networking

What is Peer-to-Peer Networking?
Peer to peer networks share responsibility for processing data among all of the connected devices. Peer-to-peer networking (also known simply as peer networking) differs from client-server networking in several respects.

How is Network Performance Measured?
The performance or "speed" of a computer network is normally measured in units of bits per second (bps). This quantity can represent either an actual data rate or a theoretical limit to available network bandwidth. The related units of Kbps, Mbps, Gbps represent increasingly larger multiples of bps.

Question: What is (Wireless / Computer) Networking?
Answer: In the world of computers, networking is the practice of linking two or more computing devices together for the purpose of sharing data. Networks are built with a mix of computer hardware and computer software.
Question: What Is the Difference Between Bits and Bytes?
The terms bit and byte are common in computer networking. Both terms refer to digital data transmitted over a network connection. For example, bits and bytes both may represent network addresses or port numbers.
Answer:
A bit is a single numeric value, either '1' or '0', that encodes a single unit of digital information. A byte is a sequence of bits; usually eight bits equal one byte.
For example, in Internet Protocol (IP) networking, IP addresses contain 32 bits or 4 bytes. The bits encode the network address so that it can be shared on the network. The bytes divide the bits into groups.
The IP address 192.168.0.1, for instance, is encoded with the following bits and bytes:
11000000 10101000 00000000 00000001
Bits are grouped into bytes to, generally speaking, increase the efficiency of computer hardware, including network equipment, disks and memory.

Client-Server vs Peer-to-Peer and Other Models
The client-server model was originally developed to allow more users to share access to database applications. Compared to the mainframe approach, client-server offers improved scalability because connections can be made as needed rather than being fixed. The client-server model also supports modular applications that can make the job of creating software easier. In so-called "two-tier" and "three-tier" types of client-server systems, software applications are separated into modular pieces, and each piece is installed on clients or servers specialized for that subsystem.
Client-server is just one approach to managing network applications The primary alternative, peer-to-peer networking, models all devices as having equivalent capability rather than specialized client or server roles. Compared to client-server, peer to peer networks offer some advantages such as more flexibility in growing the system to handle large number of clients. Client-server networks generally offer advantages in keeping data secure.
What is VPN?

VPN supplies network connectivity over a possibly long physical distance. In this respect, a VPN is a form of Wide Area Network (WAN). VPNs enable file sharing, video conferencing and similar network services. Virtual private networks generally don't provide any new functionality that isn't already offered through alternative mechanisms, but a VPN implements those services more efficiently / cheaply in most cases.
A key feature of a VPN is its ability to work over both private networks as well as public networks like the Internet. Using a method called tunneling, a VPN use the same hardware infrastructure as existing Internet or intranet links. VPN technologies includes various security mechanisms to protect the virtual, private connections.
Q. "What is the difference between a hub and a switch? Which one is better for my network?"
A. A switch is effectively a higher-performance alternative to a hub. This article describes hubs in more detail. People tend to benefit from a switch over a hub if their home network has four or more computers, or if they want to use their home network for applications that generate significant amounts of network traffic, like multiplayer games or heavy music file sharing. In most other cases, home networkers will not notice an appreciable difference between a hub and switch (hubs do cost slightly .. . Technically speaking, hubs operate using a broadcast model and switches operate using a virtual circuit model. When four computers are connected to a hub, for example, and two of those computers communicate with each other, hubs simply pass through all network traffic to each of the four computers. Switches, on the other hand, are capable of determining the destination of each individual traffic element (such as an Ethernet frame) and selectively forwarding data to the one computer that actually needs it. By generating less network traffic in delivering messages, a switch performs better than a hub on busy networks.

What is a port number?

Q. "There are many definitions for the term port number, but they don't help me form a picture in my head. I know every Internet service listens on a particular server port number - like 80 for HTTP, 21 for FTP. But how exactly do port numbers work?"
A. Where port numbers are concerned, consider an analogy between the computer and the telephone. Port numbers and their associated network addresses work something like telephone numbers and associated extensions... (see belo. One can think of a network address - say, an IP address - like a telephone number. The IP address gives a unique number for reaching someone on the Internet, just as a telephone number allows you to reach a specific destination on the public telephone network.
(Note that telephone numbers have additional features that IP addresses do not have, such as area codes that provide some geographic information about a location.)
Many larger organizations set up their telephone networks to use phone extensions. These extensions are typically 2-5 digit numbers that identify an individual phone within the organization. But from the outside, all of these extensions are associated with a single telephone number (in the USA, often a 1-800 or 1-888 number).
A network port number functions similarly to a telephone extension. Taken together with a network address, a port number identifies both a computer and also a "channel" within that computer where network communication will take place. Just as different organizations may use the same extension numbers "inside" their primary phone number, different computers use the same set of port numbers.

What is the difference between solid and stranded CAT5 cable, and which one is better?
Q. "We recently bought 250 ft. (76 m) of CAT5 cable for our home networking project. When it arrived, the box said that the cable was 'stranded.' We were wondering if we may have bought the wrong type and should we have bought solid instead?"
A.Generally speaking, stranded CAT5 is more commonly used in home networking than solid CAT5. Stranded cables are more flexible than solid ones, that allows cabling to be moved around the home without fear of cracking the wires inside. Stranded cables also cost less than solid cables... (see below.. . Solid CAT5 cables have their advantages, too. Theoretically speaking, solid CAT5 supports a higher level of performance due to its superior electrical properties compared to stranded cables, though the benefit can be negligible in practice. Businesses that are accustomed to wiring inside office walls or under floors to fixed locations, and are willing to pay more for the possibility of improved network performance, often prefer solid cabling.
Finally, when working with CAT5 cable, it's important to know that several different types of RJ-45 connectors exist. One type, designed for use with stranded cables, generally is incompatible with solid cable. Other types of RJ-45 connectors may work with both stranded and solid CAT5.
Q. "What is the 5-4-3-2-1 rule of network design?"
A. The 5-4-3-2-1 rule embodies a simple recipe for network design. It may not be easy to find examples in practice, but this rule neatly ties together several important elements of design theory... (See below) .. . To understand this rule, it's first necessary to understand the concepts of collision domains and propagation delay. Collision domains are portions of a network. When a network packet is transmitted over Ethernet, for example, it is possible for another packet from a different source to be transmitted close enough in time to the first packet to cause a collision on the wire. The total range over which a packet can travel and potentially collide with another is its collision domain.
Propagation delays are a property of the physical medium (e.g., Ethernet). Propagation delays help determine how much of a time difference between the sending of two packets on a collision domain is "close enough" to actually cause a collision. The greater the propagation delay, the increased likelihood of collisons.
The 5-4-3-2-1 rule limits the range of a collision domain by limiting the propagation delay to a "reasonable" amount of time. The rule breaks down as follows:
5 - the number of network segments
4 - the number of repeaters needed to join the segments into one collision domain
3 - the number of network segments that have active (transmitting) devices attached
2 - the number of segments that do not have active devices attached
1 - the number of collision domains
Because the last two elements of the recipe follow naturally from the others, this rule is sometimes also known as the "5-4-3" rule for short.
Q. "What is a good computer networking project for a high school or college student?"
A. One good project in networking is home LAN setup. Show how multiple computers can be networked using direct cable connections, hubs and switches, or in other ways. College students might consider building an intranet that consists of a LAN and groupware or other network software for business. High school students can use their LAN to demonstrate chat programs, Napster, or network games... (See below) .. . For college students, Artifical Intelligence (AI) projects can demonstrate "cool" uses of computer networking. One can build a robot, for example, and network it to a controlling computer.
Students can set up a Web server like Apache and show how it is configured and managed.
Finally, computer viruses make for some interesting networking projects. One can demonstrate how these viruses are created, transmitted through email (primarily), and detected and cleaned with antivirus software utilities.
Q. "Is there a way to transfer files between two PCs using a cable called a null modem cable?"
A. Yes! This cable allows you to set up file sharing between two PCs by connecting their serial or parallel ports. The details of the setup depend on which operating
systems run on the PCs... (see below)... . The term "null modem" comes from the ability of the cable to provide networking at relatively slow speeds without actually using modems.
Null modem cables do not require computers to possess Ethernet cards. For this reason, they are a very popular (though not the only) form of Direct Cable Connection (DCC) possible between PCs.

Q. "Is there any difference between bps (small 'b') and Bps (capital 'b')?"

A. The term "bps" specifies network bandwidth in bits per second. The term "Bps" specifies network bandwidth in bytes per second.

VoIP - Voice Over IP - Internet Telephony
With VoIP - voice over IP - service, consumers enjoy the convenience of a cell phone with the benefits of full Internet connectivity for telephony. The same Internet Protocol (IP) technology used in conventional data networks can be used to transmit voice and fax.
WINS - Windows Internet Naming Service
WINS supports name resolution, the automated conversion of computer names to network addresses, for Windows networks. WINS converts NetBIOS names to IP addresses.
NetBIOS - Network Basic Input/Output System
NetBIOS is a protocol for computer communication services on local networks used by Microsoft Windows.
Microsoft Internet Information Server (IIS)
Microsoft IIS implements Web server functionality that integrates with other Microsoft server networking technologies.
DNS servers
Main article: Name server
The Domain Name System is maintained by a distributed database system, which uses the client-server model. The nodes of this database are the name servers. Each domain or subdomain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains subordinate to it. The top of the hierarchy is served by the root nameservers: the servers to query when looking up (resolving) a top-level domain name (TLD).
DHCP



1. What is DHCP?
DHCP stands for "Dynamic Host Configuration Protocol".
2. What is DHCP's purpose?
DHCP's purpose is to enable individual computers on an IP network to extract their configurations from a server (the 'DHCP server') or servers, in particular, servers that have no exact information about the individual computers until they request the information. The overall purpose of this is to reduce the work necessary to administer a large IP network. The most significant piece of information distributed in this manner is the IP address.
3. Can DHCP work with Appletalk or IPX?
No, it is too tied to IP. Furthermore, they don't need it since they have always had automated mechanisms for assigning their own network addresses.
4. Who Created It? How Was It Created?
DHCP was created by the Dynamic Host Configuration Working Group of the Internet Engineering Task Force (IETF; a volunteer organization which defines protocols for use on the Internet). As such, it's definition is recorded in an Internet RFC and the Internet Activities Board (IAB) is asserting its status as to Internet Standardization. As of this writing (June 1998), DHCP is an Internet Draft Standard Protocol and is Elective. BOOTP is an Internet Draft Standard Protocol and is Recommended. For more information on Internet standardization, see RFC2300 (May 1998)
5. How is it different than BOOTP or RARP?
DHCP is based on BOOTP and maintains some backward compatibility. The main difference is that BOOTP was designed for manual pre-configuration of the host information in a server database, while DHCP allows for dynamic allocation of network addresses and configurations to newly attached hosts. Additionally, DHCP allows for recovery and reallocation of network addresses through a leasing mechanism.
RARP is a protocol used by Sun and other vendors that allows a computer to find out its own IP number, which is one of the protocol parameters typically passed to the client system by DHCP or BOOTP. RARP doesn't support other parameters and using it, a server can only serve a single LAN. DHCP and BOOTP are designed so they can be routed.
6. How is it different than VLANs?
DHCP and VLANs, which are very different in concept, are sometimes cited as different solutions to the same problem. While they have a goal in common (easing moves of networked computers), VLANs represent a more revolutionary change to a LAN than DHCP. A DHCP server and forwarding agents can allow you to set things up so that you can unplug a client computer from one network or subnet and plug it into another and have it come alive immediately, it having been reconfigured automatically. In conjunction to Dynamic DNS, it could automatically be given its same name in its new place. VLAN-capable LAN equipment with dynamic VLAN assignment allows you to configure things so a client computer can be plugged into any port and have the same IP number (as well as name) and be on the same subnet. The VLAN-capable network either has its own configuration that lists which MAC addresses are to belong to each VLAN, or it makes the determination from the source IP address of the IP packets that the client computer sends. Some differences in the two approaches:
o DHCP handles changes by reconfiguring the client while a VLAN-capable network handles it by reconfiguring the network port the client is moved to.
o DHCP dynamic reconfiguration requires a DHCP server, forwarding agent in each router, and DHCP capability in each client's TCP/IP support. The analogous capability in VLANs requires that all hubs throughout the network be VLAN-capable, supporting the same VLAN scheme. To this point VLAN support is proprietary with no vendor interoperability, but standards are being developed.
o DHCP can configure a new client computer for you while a VLAN-capable network can't.
o DHCP is generally aimed at giving "easy moves" capability to networks that are divided into subnets on a geographical basis, or on separate networks. VLANs are generally aimed at allowing you to set up subnets on some basis other than geographical, e.g. instead of putting everyone in one office on the same subnet, putting each person on a subnet that has access to the servers that that person requires.
There is an issue with trying to use DHCP (or BOOTP) and VLANs at the same time, in particular, with the scheme by which the VLAN-capable network determines the client's VLAN based upon the client computer's source IP address. Doing so assumes the client computer is already configured, which precludes the use of network to get the configuration information from a DHCP or BOOTP server.
7. What protocol and port does DHCP use?
DHCP, like BOOTP runs over UDP, utilizing ports 67 and 68.
8. What is an IP address?
An IP address (also called an IP number) is a number (typically written as four numbers separated by periods, i.e. 107.4.1.3 or 84.2.1.111) which uniquely identifies a computer that is making use of the Internet. It is analogous to your telephone number in that the telephone number is used by the telephone network to direct calls to you. The IP address is used by the Internet to direct data to your computer, e.g. the data your web browser retrieves and displays when you surf the net. One task of DHCP is to assist in the problem of getting a functional and unique IP number into the hands of the computers that make use of the Internet.
9. What is a MAC address?
A MAC address (also called an Ethernet address or an IEEE MAC address) is a number (typically written as twelve hexadecimal digits, 0 through 9 and A through F, or as six hexadecimal numbers separated by periods or colons, i.e. 0080002012ef, 0:80:0:2:20:ef) which uniquely identifes a computer that has an Ethernet interface. Unlike the IP number, it includes no indication of where your computer is located. In DHCP's typical use, the server uses a requesting computer's MAC address to uniquely identify it.
10. What is a DHCP lease?
A DHCP lease is the amount of time that the DHCP server grants to the DHCP client permission to use a particular IP address. A typical server allows its administrator to set the lease time.
11. What is a Client ID?
What is termed the Client ID for the purposes of the DHCP protocol is whatever is used by the protocol to identify the client computer. By default, DHCP implementations typically employ the client's MAC address for this purpose, but the DHCP protocol allows other options. Some DHCP implementations have a setup option to specify the client ID you want. One alternative to the MAC address is simply a character string of your choice. In any case, in order for DHCP to function, you must be certain that no other client is using the client ID you choose, and you must be sure the DHCP server will accept it.
12. Why shouldn't clients assign IP numbers without the use of a server?
It is theoretically possible to develop software for client-machines that finds an unused address by picking them out of the blue and broadcasting a request of all the other client machines to see if they are using them. Appletalk is designed around this idea, and Apple's MacTCP can be configured to do this for IP. However, this method of IP address assignment has disadvantages.
1. A computer that needs a permanently-assigned IP number might be turned off and lose its number to a machine coming up. This has problems both for finding services and for security.
2. A network might be temporarily divided into two non-communicating networks while a network component is not functioning. During this time, two different client-machines might end up claiming the same IP number. When the network comes back, they start malfunctioning.
3. If such dynamic assignment is to be confined to ranges of IP addresses, then the ranges are configured in each desktop machine rather than being centrally administered. This can lead both to hidden configuration errors and to difficulty in changing the range. Another problem with the use of such ranges is keeping it easy to move a computer from one subnet to another.
13. Can DHCP support statically defined addresses?
Yes. At least there is nothing in the protocol to preclude this and one expects it to be a feature of any DHCP server. This is really a server matter and the client should work either way. The RFC refers to this as manual allocation.
14. How does DHCP and BOOTP handle multiple subnets?
For the situations where there is more than one LAN, each with its own subnet number, there are two ways. First of all, you can set up a seperate server on each subnet. Secondly, a feature of some routers known as "BOOTP forwarding" to forward DHCP or BOOTP requests to a server on another subnet and to forward the replies back to the client. The part of such a router (or server acting as a router) that does this is called a "BOOTP forwarding agent". Typically you have to enable it on the interface to the subnet to be served and have to configure it with the IP address of the DHCP or BOOTP server. On a Cisco router, the address is known as the "UDP Helper Address".
15. Can a BOOTP client boot from a DHCP server?
Only if the DHCP server is specifically written to also handle BOOTP queries.
16. Can a DHCP client boot from a BOOTP server?
Only if the DHCP client were specifically written to make use of the answer from a BOOTP server. It would presumably treat a BOOTP reply as an unending lease on the IP address.
In particular, the TCP/IP stack included with Windows 95 does not have this capability.
17. Is a DHCP server "supposed to" be able to support a BOOTP client?
The RFC on such interoperability (1534) is clear: "In summary, a DHCP server: ... MAY support BOOTP clients," (section 2). The word "MAY" indicates such support, however useful, is left as an option.
A source of confusion on this point is the following statement in section 1.5 of RFC 1541: "DHCP must provide service to existing BOOTP clients." However, this statement is one in a list of "general design goals for DHCP", i.e. what the designers of the DHCP protocol set as their own goals. It is not in a list of requirements for DHCP servers.
18. Is a DHCP client "supposed to" be able to use a BOOTP server?
The RFC on such interoperability (1534) is clear: "A DHCP client MAY use a reply from a BOOTP server if the configuration returned from the BOOTP server is acceptable to the DHCP client." (section 3). The word "MAY" indicates such support, however useful, is left as an option.
19. Can a DHCP client or server make a DNS server update the client's DNS entry to match the client's dynamically assigned address?
RFCs 2136 and 2137 indicate a way in which DNS entries can be updated dynamically. Using this requires a DNS server that supports this feature and a DHCP server that makes use of it. The RFCs are very recent (as of 5/97) and implementations are few. In the mean time, there are DNS and DHCP servers that accomplish this through proprietary means.
20. Can a DHCP server back up another DHCP server?
You can have two or more servers handing out leases for different addresses. If each has a dynamic pool accessible to the same clients, then even if one server is down, one of those clients can lease an address from the other server.
However, without communication between the two servers to share their information on current leases, when one server is down, any client with a lease from it will not be able to renew their lease with the other server. Such communication is the purpose of the "server to server protocol" (see next question). It is possible that some server vendors have addressed this issue with their own proprietary server-to-server communication.
21. When will the server to server protocol be defined?
The DHC WG of the IETF is actively investigating the issues in inter-server communication. The protocol should be defined "soon".
22. Is there a DHCP mailing list?
There are several:
List Purpose
---- -------
dhcp-v4@bucknell.edu General discussion: a good list for
server administrators.
dhcp-bake@bucknell.edu DHCP bakeoffs
dhcp-impl@bucknell.edu Implementations
dhcp-serve@bucknell.edu Server to server protocol
dhcp-dns@bucknell.edu DNS-DHCP issues
dhcp-v6@bucknell.edu DHCP for IPv6

The lists are run by listserv@bucknell.edu which can be used to subscribe and sign off. Archives for the dhcp-v4 list (which used to be called the host-conf list) are stored at ftp://ftp.bucknell.edu/pub/dhcp/.
23. In a subnetted environment, how does the DHCP server discover what subnet a request has come from?
DHCP client messages are sent to off-net servers by DHCP relay agents, which are often a part of an IP router. The DHCP relay agent records the subnet from which the message was received in the DHCP message header for use by the DHCP server.
Note: a DHCP relay agent is the same thing as a BOOTP relay agent, and technically speaking, the latter phrase is correct.
24. If a single LAN has more than one subnet number, how can addresses be served on subnets other than the primary one?
A single LAN might have more than one subnet number applicable to the same set of ports (broadcast domain). Typically, one subnet is designated as primary, the others as secondary. A site may find it necessary to support addresses on more than one subnet number associated with a single interface. DHCP's scheme for handling this is that the server has to be configured with the necessary information and has to support such configuration & allocation. Here are four cases a server might have to handle:
0. Dynamic allocation supported on secondary subnet numbers on the LAN to which the server is attached.
1. Dynamic allocation supported on secondary subnet numbers on a LAN which is handled through a DHCP/BOOTP Relay. In this case, the DHCP/BOOTP Relay sends the server a gateway address associated with the primary subnet and the server must know what to do with it.
The other two cases are the same capabilities during manual allocation. It is possible that a particular server-implementation can handle some of these cases, but not all of them. See section below listing the capabilities of some servers.
25. If a physical LAN has more than one logical subnet, how can different groups of clients be allocated addresses on different subnets?
One way to do this is to preconfigure each client with information about what group it belongs to. A DHCP feature designed for this is the user class option. To do this, the client software must allow the user class option to be preconfigured and the server software must support its use to control which pool a client's address is allocated from.
26. Where is DHCP defined?
In Internet RFCs.
RFC 2131
R. Droms, "Dynamic Host Configuration Protocol", 3/97. Supersedes RFC 1541 and RFC 1531. [Note that some of the references in this FAQ are to RFC 1541: I'll update them when I get a chance. -- Author]
RFC 1534
R. Droms, "Interoperation Between DHCP and BOOTP", 10/08/1993.
RFC 2132
S. Alexander, R. Droms, "DHCP Options and BOOTP Vendor Extensions", 3/97. Supersedes RFC 1533.
Some websites with copies of RFCs:
http://info.internet.isi.edu/1s/in-notes/rfc/
http://www.cis.ohio-state.edu/hypertext/information/rfc.html
http://www.pmg.lcs.mit.edu/rfc.html
27. What other sources of information are available?
See the dhcp-v4 mailing list mentioned above as well as its archives.
DHCP - Dynamic Host Configuration Protocol
http://www.eg.bucknell.edu/~droms/dhcp/
Problems and Solutions of DHCP: Experiences with DHCP implementation and Operation
A. Tominaga, O. Nakamura, F. Teraoka, J. Murai. http://info.isoc.org/HMP/PAPER/127/html/paper.html
DHCP Resources
Alan Dobkin. http://NWS.CC.Emory.Edu/WebStaff/Alan/Net-Man/Computing/DHCP/
DHCP Reading Room
Eric Hall. http://www.ehsco.com/reading/dhcp.html
Internet Drafts
Internet drafts are works in progress intended to update the current RFCs or specify additional functionality, and sometimes there is one or more draft related to DHCP. All Internet Drafts are available from various sites: the US East Cost site is ftp://ds.internic.net/internet-drafts/; a web site is http://ds.internic.net/ds/dsintdrafts.html. The DHCP-related drafts currently have filenames of the form "draft-ietf-dhc-SOMETHING". These DHCP-related drafts are also stored at ftp://ftp.bucknell.edu/pub/dhcp/, and are available through http://www.eg.bucknell.edu/~droms/dhcp/. I cannot be more specific about the documents because they are by their nature temporary.
"DHCP Clients: Do They Really Work?"
Eric Hall. Network Computing, Vol. 7, No. 7, May 1, 1996, pp. 114-120. Reviews DHCP-client-function of some popular Windows IP stacks. http://www.ehsco.com/reading/19960515ncw2.html
"The Heaven And Hell Of DHCP Servers"
Eric Hall. Network Computing, Vol. 7, No. 8, May 15, 1996, pp. 118-121. Reviews DHCP servers. http://www.ehsco.com/reading/19960515ncw1.html
28. Can DHCP support remote access?
PPP has its own non-DHCP way in which communications servers can hand clients an IP address called IPCP (IP Control Protocol) but doesn't have the same flexibility as DHCP or BOOTP in handing out other parameters. Such a communications server may support the use of DHCP to acquire the IP addresses it gives out. This is sometimes called doing DHCP by proxy for the client. I know that Windows NT's remote access support does this.
A feature of DHCP under development (DHCPinform) is a method by which a DHCP server can supply parameters to a client that already has an IP number. With this, a PPP client could get its IP number using IPCP, then get the rest of its parameters using this feature of DHCP.
SLIP has no standard way in which a server can hand a client an IP address, but many communications servers support non-standard ways of doing this that can be utilized by scripts, etc. Thus, like communications servers supporting PPP, such communications servers could also support the use of DHCP to acquire the IP addressees to give out.
The DHCP protocol is capable of allocating an IP address to a device without an IEEE-style MAC address, such as a computer attached through SLIP or PPP, but to do so, it makes use of a feature which may or may not be supported by the DHCP server: the ability of the server to use something other than the MAC address to identify the client. Communications servers that acquire IP numbers for their clients via DHCP run into the same roadblock in that they have just one MAC address, but need to acquire more than one IP address. One way such a communications server can get around this problem is through the use of a set of unique pseudo-MAC addresses for the purposes of its communications with the DHCP server. Another way (used by Shiva) is to use a different "client ID type" for your hardware address. Client ID type 1 means you're using MAC addresses. However, client ID type 0 means an ASCII string.
29. Can a client have a home address and still float?
There is nothing in the protocol to keep a client that already has a leased or permanent IP number from getting a(nother) lease on a temporary basis on another subnet (i.e., for that laptop which is almost always in one office, but occasionally is plugged in in a conference room or class room). Thus it is left to the server implementation to support such a feature. I've heard that Microsoft's NT-based server can do it.
30. How can I relay DHCP if my router does not support it?
A server on a net(subnet) can relay DHCP or BOOTP for that net. Microsoft has software to make Windows NT do this.
31. How do I migrate my site from BOOTP to DHCP?
I don't have an answer for this, but will offer a little discussion. The answer depends a lot on what BOOTP server you are using and how you are maintaining it. If you depend heavily on BOOTP server software to support your existing clients, then the demand to support clients that support DHCP but not BOOTP presents you with problems. In general, you are faced with the choice:
0. Find a server that is administered like your BOOTP server only that also serves DHCP. For example, one popular BOOTP server, the CMU server, has been patched so that it will answer DHCP queries.
1. Run both a DHCP and a BOOTP server. It would be good if I could find out the gotcha's of such a setup.
2. Adapt your site's administration to one of the available DHCP/BOOTP servers.
3. Handle the non-BOOTP clients specially, e.g. turn off DHCP and configure them statically: not a good solution, but certainly one that can be done to handle the first few non-BOOTP clients at your site.
32. Can you limit which MAC addresses are allowed to roam?
Sites may choose to require central pre-configuration for all computers that will be able to acquire a dynamic address. A DHCP server could be designed to implement such a requirement, presumably as an option to the server administrator. See section below on servers that implement this.
33. Is there an SNMP MIB for DHCP?
There is no standard MIB; creating one is on the list of possible activities of the DHCP working group. It is possible that some servers implement private MIBs.
34. What is DHCP Spoofing?
Ascend Pipeline ISDN routers (which attach Ethernets to ISDN lines) incorporate a feature that Ascend calls "DHCP spoofing" which is essentially a tiny server implementation that hands an IP address to a connecting Windows 95 computer, with the intention of giving it an IP number during its connection process.
35. How long should a lease be?
I've asked sites about this and have heard answers ranging from 15 minutes to a year. Most administrators will say it depends upon your goals, your site's usage patterns, and service arrangements for your DHCP server.
A very relevant factor is that the client starts trying to renew the lease when it is halfway through: thus, for example, with a 4 day lease, the client which has lost access to its DHCP server has 2 days from when it first tries to renew the lease until the lease expires and the client must stop using the network. During a 2-day outage, new users cannot get new leases, but no lease will expire for any computer turned on at the time that the outage commences.
Another factor is that the longer the lease the longer time it takes for client configuration changes controlled by DHCP to propogate.
Some relevant questions in deciding on a lease time:
Do you have more users than addresses?
If so, you want to keep the lease time short so people don't end up sitting on leases. Naturally, there are degrees. In this situation, I've heard examples cited of 15 minutes, 2 hours, and 2 days. Naturally, if you know you will have 20 users using 10 addresses in within a day, a 2 day lease is not practical.
Are you supporting mobile users?
If so, you may be in the situation of having more users than addresses on some particular IP number range. See above.
Do you have a typical or minimum amount of time that you are trying to support?
If your typical user is on for an hour at minimum, that suggest a hour lease at minimum.
How many clients do you have and how fast are the communications lines over which the DHCP packets will be run?
The shorter the lease, the higher the server and network load. In general, a lease of at least 2 hours is long enough that the load of even thousands of clients is negligible. For shorter leases, there may be a point beyond which you will want to watch the load. Note that if you have a communication line down for a long enough time for the leases to expire, you might see an unusually high load it returns. If the lease-time is at least double the communication line outage, this is avoided.
How long would it take to bring back up the DHCP server, and to what extent can your users live without it?
If the lease time is at least double the server outage, then running clients who already have leases will not lose them. If you have a good idea of your longest likely server outage, you can avoid such problems. For example, if your server-coverage is likely to recover the server within three hours at any time that clients are using their addresses, then a six hour lease will handle such an outage. If you might have a server go down on Friday right after work and may need all Monday's work-day to fix it, then your maximum outage time is 3 days and a 6-day lease will handle it.
Do you have users who want to tell other users about their IP number?
If your users are setting up their own web servers and telling people how to get to them either by telling people the IP number or through a permanent DNS entry, then they are looking for an IP number that won't be changing. While some sites would manually allocate any address that people expected to remain stable, other sites want to use DHCP's ability to automate distribution of relatively permanent addresses. The relevant time is the maximum amount of time that you wish to allow the user to keep their machine turned off yet keep their address. For example, in a university, if students might have their computers turned off for as long as three weeks between semesters, and you wish them to keep their IP address, then a lease of six weeks or longer would suffice.
Some examples of lease-times that sites have used & their rationals:
15 minutes
To keep the maximum number of addresses free for distribution in cases where there will be more users than addresses.
6 hours
Long enough to allow the DHCP server to be fixed, e.g. 3 hours.
12 hours
If you need to take back an address, then you know that it will only take one night for the users' lease to expire.
3 days
This is apparently Microsoft's default, thus many sites use it.
6 days
Long enough that a weekend server outage that gets fixed on Monday will not result in leases terminating.
4 months
Long enough that students can keep their IP address over the summer hiatus. I believe this rational is workable if the summer hiatus is no more than 2 months.
One year
If a user has not used their address in six months, then they are likely to be gone. Allows administrator to recover those addresses after someone has moved on.
36. How can I control which clients get leases from my server?
There is no ideal answer: you have to give something up or do some extra work.
o You can put all your clients on a subnet of your own along with your own DHCP server.
o You can use manual allocation.
o Perhaps you can find DHCP server software that allows you to list which MAC addresses the server will accept. DHCP servers that support roaming machines may be adapted to such use.
o You can use the user class option assuming your clients and server support it: it will require you to configure each of your clients with a user class name. You still depend upon the other clients to respect your wishes.
37. How can I prevent unauthorized laptops from using a network that uses DHCP for dynamic addressing?
This would have to be done using a mechanism other than DHCP. DHCP does not prevent other clients from using the addresses it is set to hand out nor can it distinguish between a computer's permanent MAC address and one set by the computer's user. DHCP can impose no restrictions on what IP address can use a particular port nor control the IP address used by any client.
38. What are the Gotcha's?
o A malicious user could make trouble by putting up an unofficial DHCP server.
 The immediate problem would be a server passing out numbers already belonging to some computer yielding the potential for two or more "innocent bystander" nodes ending up with the same IP number. Net result is problems using the nodes, possibly intermittent of one or the other is sometimes turned off.
 A lot of problems are possible if a renegade server manages to get a client to accept its lease offering, and feeds the client its own version of other booting parameters. One scenario is a client that loads its OS over the network via tftp being directed to a different file (possibly on a different server), thus allowing the perpetrator to take over the client. Given that boot parameters are often made to control many different things about the computers' operation and communication, many other scenarios are just as serious.
Note that BOOTP has the same vulnerabilities.
o The "broadcast flag": DHCP includes a way in which client implementations unable to receive a packet with a specific IP address can ask the server or relay agent to use the broadcast IP address in the replies (a "flag" set by the client in the requests). The definition of DHCP states that implementations "should" honor this flag, but it doesn't say they "must". Some Microsoft TCP/IP implementations used this flag, which meant in practical terms, relay agents and servers had to implement it. A number of BOOTP-relay-agent implementations (e.g. in routers) handled DHCP just fine except for the need for this feature, thus they announced new versions stated to handle DHCP.
o Some of the virtual LAN schemes, i.e., those that use the packet's IP number to decide which "virtual LAN" a client-computer is on for the purposes of TCP/IP, don't work when using DHCP to dynamically assign addresses. DHCP servers and relay agents use their knowledge of what LAN the client-station is on to select the subnet number for the client-station's new IP address whereas such switches use the subnet number sent by the client-station to decide which (virtual) LAN to put the station on.
o Routers are sometimes configured so that one LAN on one port has multiple network (or subnet) numbers. When the router is relaying requests from such a LAN to the DHCP server, it must pass along as IP number that is associated with one of the network (or subnet) numbers. The only way the DHCP server can allocate addresses on one of the LAN's other network (or subnet) numbers is if the DHCP server is specifically written to have a feature to handle such cases, and it has a configuration describing the situation.
o The knowledge that a particular IP number is associated with a particular node is often used for various functions. Examples are: for security purposes, for network management, and even for identifying resources. Furthermore, if the DNS's names are going to identify IP numbers, the numbers, the IP numbers have to be stable. Dynamic configuration of the IP numbers undercuts such methods. For this reason, some sites try to keep the continued use of dynamically allocatable IP numbers to a minimum.
o With two or more servers serving a LAN, clients that are moved around (e.g. mobile clients) can end up with redundant leases. Consider a home site with two DHCP servers, a remote site with DHCP services, and a mobile client. The client first connects to the home site and receives an address from one of the two serves. He/she then travels to the remote site (without releasing the lease at the home site) and attempts to use the acquired address. It is of course NAK'ed and the client receives an address appropriate for the remote site. The client then returns home and tries to use the address from the remote site. It is NAK'ed but now the client broadcasts a DHCPDISCOVER to get a address. The server that holds the previous lease will offer the address back to the client but there is no guarantee that the client will accept that address; consequently, it is possible for the client to acquire an address on the other server and therefore have two leases within the site. The problem can be solved by using only one server per subnet/site and can be mitigated by short lease lengths. But in a very mobile environment, it is possible for these transient clients to consume more than their fair share of addresses.
o If departments, offices, or individuals run DHCP servers with their own small address pools on LANs shared by other departments, offices, or individuals, they can find that their addresses are being used by anyone on the LAN that happens to set their IP configuration to use DHCP.
o An easy mistake to make in setting up a DHCP server is to fail to set all the necessary global parameters. This can result in some functions working while others are not, or functions working when the client is set up manually, but failing to work when set to use DHCP.
o Long leases can be disadvantageous in cases where you need to change a configuration parameter or withdraw an address from use. The length of the lease can mean the difference between having to go to every affected client and rebooting it, or merely waiting a certain amount of time for the leases to be renewed. (Note: one workaround is to fool with the client computer's clock).
Network address translation
In computer networking, network address translation (NAT) is the process of modifying network address information in datagram packet headers while in transit across a traffic routing device for the purpose of remapping a given address space into another.
Most often today, NAT is used in conjunction with network masquerading (or IP masquerading) which is a technique that hides an entire address space, usually consisting of private network addresses (RFC 1918), behind a single IP address in another, often public address space. This mechanism is implemented in a routing device that uses stateful translation tables to map the "hidden" addresses into a single address and then rewrites the outgoing Internet Protocol (IP) packets on exit so that they appear to originate from the router. In the reverse communications path, responses are mapped back to the originating IP address using the rules ("state") stored in the translation tables. The translation table rules established in this fashion are flushed after a short period without new traffic refreshing their state.
As described, the method only allows transit traffic through the router when it is originating in the masqueraded network, since this establishes the translation tables. However, most NAT devices today allow the network administrator to configure translation tables entries for permanent use. This feature is often referred to as "static NAT" or port forwarding and allows traffic originating in the 'outside' network to reach designated hosts in the masqueraded network.
Because of the popularity of this technique, see below, the term NAT has become virtually synonymous with the method of IP masquerading.
Network address translation has serious consequences (see below, Drawbacks & Benefits) on the quality of Internet connectivity and requires careful attention to the details of its implementation. As a result, many methods have been devised to alleviate the issues encountered. See article on NAT traversal.
Remote Access Service
Remote Access Services (RAS) refers to any combination of hardware and software to enable the remote access to tools or information that typically reside on a network of IT devices.
Originally coined by Microsoft when referring to their built-in NT remote access tools, RAS was a service provided by Windows NT which allows most of the services which would be available on a network to be accessed over a modem link. The service includes support for dialup and logon, and then presents the same network interface as the normal network drivers (albeit slightly slower). It is not necessary to run Windows NT on the client - there are client versions for other Windows operating systems.
A feature built into Windows NT that enables users to log into an NT-based LAN using a modem, X.25 connection or WAN link. RAS works with several major network protocols, including TCP/IP, IPX, and NBF.
To use RAS from a remote node, you need a RAS client program, which is built into most versions of Windows, or any PPP client software. For example, most remote control programs work with RAS.
Over the years, many vendors have provided both hardware and software solutions to gain remote access to various types of networked information. In fact, most modern routers include a basic RAS capability that can be enabled for any dial-up interface.
Remote Installation Services
On Windows 2003, two services are required to provide Remote Installation Services: DHCP and Remote Installation Service. The Remote Installation Server doubles as a proxy DHCP server to provide Boot Server and Filename instructions to clients. Remote Installation Service utilizes UDP port 4011[2] to provide clients the contents of each page the OS Chooser displays. Additionally, this service can provide drivers to clients; it is often used to provide the workstation's network card driver, which is required to launch the OS Chooser and mount the share where images are stored.