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TCP/IP FAQ; Frequently Asked Questions (1999-09) Part 1 of 2

Part 1 of a 2-part informational posting that contains responses to common questions on basic TCP/IP network protocols and applications.
Archive-name:      internet/tcp-ip/tcp-ip-faq/part1
Version:           5.15
Last-modified:     1999-09-06 20:11:43
Posting-Frequency: monthly (first Friday)
Maintainer: (Mike Oliver)

TCP/IP Frequently Asked Questions

Part 1: Introduction and Fundamental Protocols

This is Part 1 of the Frequently Asked Questions (FAQ) list for the
comp.protocols.tcp-ip Usenet newsgroup. The FAQ provides answers to a
selection of common questions on the various protocols (IP, TCP, UDP,
ICMP and others) that make up the TCP/IP protocol suite. It is posted
to the news.answers, comp.answers and comp.protocols.tcp-ip newsgroups
on or about the first Friday of every month.

The FAQ is posted in two parts. Part 1 contains answers to general
questions and questions that concern the fundamental components of the
suite. Part 2 contains answers to questions concerning common
applications that depend on the TCP/IP suite for their network

Comments on this document can be emailed to the FAQ maintainer at

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Table of Contents

FAQ Part 1: Introduction and Fundamental Protocols


  1. Where can I find an up-to-date copy of this FAQ?
  2. Who wrote this FAQ?

About TCP/IP

  1. What is TCP/IP?
  2. How is TCP/IP defined?
  3. Where can I find RFC's?
  4. How do I find the right RFC?

About IP

  1. What is IP?
  2. How is IP carried on a network?
  3. Does IP Protect Data on the Network?
  4. What is ARP?
  5. What is IPv6?
  6. What happened to IPv5?
  7. What is the 6bone?
  8. What is the MBONE?
  9. What is IPsec?

About TCP

  1. What is TCP?
  2. How does TCP try to avoid network meltdown?
  3. How do applications coexist over TCP and UDP?
  4. Where do I find assigned port numbers?

About UDP

  1. What is UDP?

About ICMP

  1. What is ICMP?

TCP/IP Network Operations

  1. How can I measure the performance of an IP link?
  2. What IP addresses should I assign to machines on a private
  3. Can I set up a gateway to the Internet that translates IP
     addresses, so that I don't have to change all our internal
     addresses to an official network?
  4. Can I use a single bit subnet?

TCP/IP Protocol Implementations

  1. Where can I find TCP/IP source code?
  2. Where can I find TCP/IP application source code?
  3. Where can I find IPv6 source code?

Further Sources of Information

  1. What newsgroups deal with TCP/IP?
  2. Are there any good books on TCP/IP?

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  1. Where can I find an up-to-date copy of this FAQ?

     You can browse a hyperlinked version of this FAQ on the World Wide
     Web at <> in the US
     (thanks to Irwin Lazar) and at
     <> in
     Israel (thanks to Uri Raz). Links to RFC's from Irwin's site refer
     to the ISI RFC repository in the US, while links to RFC's from
     Uri's site refer to the RFC repository at Imperial College in the
     UK. Use whichever gives you better response time.

     The current version of this FAQ is posted on a monthly basis to
     the news.answers, comp.answers and comp.protocols.tcp-ip

     A plaintext copy of the most recently posted version of the FAQ is
     available by anonymous FTP from

  2. Who wrote this FAQ?

     This FAQ was compiled from Usenet postings and email contributions
     made by many people, including: Rui Duarte Tavares Bastos, Mark
     Bergman, Stephane Bortzmeyer, Rodney Brown, Dr. Charles E.
     Campbell Jr., James Carlson, Phill Conrad, Alan Cox, Michael
     Hunter, Jay Kreibrich, William Manning, Barry Margolin, Vic
     Metcalfe, Jim Muchow, George V. Neville-Neil, Dang Thanh Ngan,
     Subu Rama, Uri Raz, and W. Richard Stevens.

     The FAQ is currently maintained by Mike Oliver. Comments,
     criticisms and contributions should be mailed to
     <>. Please do not send TCP/IP questions to
     this address; it is intended only for FAQ issues. If you have a
     question that is not already answered by the material in this FAQ
     you will get a much faster (and probably more accurate) response
     by posting the question to the comp.protocols.tcp-ip newsgroup
     than you will by sending it to the FAQ maintainer.

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About TCP/IP

  1. What is TCP/IP?

     TCP/IP is a name given to the collection (or suite) of networking
     protocols that have been used to construct the global Internet.
     The protocols are also referred to as the DoD (dee-oh-dee) or
     Arpanet protocol suite because their early development was funded
     by the Advanced Research Projects Agency (ARPA) of the US
     Department of Defense (DoD).

     The TCP/IP name is taken from two of the fundamental protocols in
     the collection, IP and TCP. Other core protocols in the suite are
     UDP and ICMP. These protocols work together to provide a basic
     networking framework that is used by many different application
     protocols, each tuned to achieving a particular goal.

     TCP/IP protocols are not used only on the Internet. They are also
     widely used to build private networks, called internets (spelled
     with a small 'i'), that may or may not be connected to the global
     Internet (spelled with a capital 'I'). An internet that is used
     exclusively by one organization is sometimes called an intranet.

  2. How is TCP/IP defined?

     All of the protocols in the TCP/IP suite are defined by documents
     called Requests For Comments (RFC's). An important difference
     between TCP/IP RFC's and other (say, IEEE or ITU) networking
     standards is that RFC's are freely available online.

     RFC's can be composed and submitted for approval by anyone.
     Standards RFC's are often the product of many weeks or months of
     discussion between interested parties designated as working
     groups, during which time drafts of the proposed RFC are
     continually updated and made available for comment. These
     discussions typically take place on open mailing lists which
     welcome input from all quarters. The RFC approval process is
     managed by the Internet Engineering Steering Group (IESG) based on
     recommendations from the Internet Engineering Task Force (IETF)
     which is a prime mover in the formation of working groups focused
     on strategic TCP/IP issues. You can find out more about IESG and
     IETF activities from the IETF home page at

     Not all RFC's specify TCP/IP standards. Some RFC's contain
     background information, some provide hints for managing an
     internet, some document protocol weaknesses in the hope that they
     might be addressed by future standards, and some are entirely

  3. Where can I find RFC's?

     The Definitive RFC Repository

     The official and definitive RFC repository is the anonymous FTP
     archive maintained by the Information Sciences Institute of the
     University of Southern California at <>.
     It is reachable via the Web at <>.

     RFC Repository Mirror Sites

     The RFC repository is mirrored at many sites on the Internet, and
     you may get a faster response from a local archive than you would
     from the often-overworked ISI site. Primary mirrors are updated at
     the same time as the ISI site. Secondary mirrors may lag by a few
     hours or days. The current primary mirror sites are:

          In the USA ...

          New Jersey:
          North Carolina:

          In Europe ...


     Secondary mirror sites are listed in a document named
     rfc-retrieval.txt which can be found alongside the RFC's
     themselves at any of the above sites.

     RFC's by Email

     If you don't have direct access to the Internet but are able to
     send and receive email then you can still get RFC's through
     various email-to-ftp gateways. For instructions on how to do this,
     send email containing the text:

          help: ways_to_get_rfcs

     to <>.

  4. How do I find the right RFC?

     There are over 2500 RFC's. Each RFC is known by a number. For
     instance, RFC 1180 presents a tutorial on TCP/IP, RFC 1920 lists
     the current standards RFC's and explains the RFC standards
     process, and RFC 1941 is a FAQ list on the topic of Internet
     deployment in educational establishments. RFC numbers are assigned
     in ascending order as each RFC is approved.

     The RFC files in the archive are named rfcNNNN.txt where NNNN is
     the number of the RFC. For instance, the text of RFC 822 is
     contained in the file named rfc822.txt. A small number of RFC's
     are also available in PostScript format, in which case a file
     named will exist in addition to the .txt file.

     Basic information (number, title, author, publication date and so
     on) on all of the RFC's is contained in the RFC index document
     named rfc-index.txt which you can find alongside the RFC's at any
     of the RFC archive sites. If you don't know which RFC's you need,
     the index is a good place to start. The index also indicates the
     current status of each RFC. The content of an RFC does not change
     once the RFC has been published, but since TCP/IP is in a constant
     state of evolution the information in one RFC is often revised,
     extended, clarified and in some cases completely superseded by
     later RFC's. Annotations in the index indicate when this is the

     If you find yourself using the index a lot then you might find it
     convenient to create your own HTML version of the index. Wayne
     Mesard has published a Perl script that takes the plaintext index
     file as input and produces an HTML version with hyperlinks to your
     chosen RFC FTP repository or to your own local RFC archive. The
     script is available at

     If you don't want to wade through the index, some sites provide
     the ability to search the RFC catalogue by keyword:

     Keyword Searches on the Web
          <> lets you search on RFC content.
          <> and
          <> let you search on words in the
          RFC title.
     Keyword Searches via gopher
          <gopher://> or
     RFC Keyword Searches via WAIS

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About IP

  1. What is IP?

     Internet Protocol (IP) is the central, unifying protocol in the
     TCP/IP suite. It provides the basic delivery mechanism for packets
     of data sent between all systems on an internet, regardless of
     whether the systems are in the same room or on opposite sides of
     the world. All other protocols in the TCP/IP suite depend on IP to
     carry out the fundamental function of moving packets across the

     In terms of the OSI networking model, IP provides a Connectionless
     Unacknowledged Network Service, which means that its attitude to
     data packets can be characterised as "send and forget". IP does
     not guarantee to actually deliver the data to the destination, nor
     does it guarantee that the data will be delivered undamaged, nor
     does it guarantee that data packets will be delivered to the
     destination in the order in which they were sent by the source,
     nor does it guarantee that only one copy of the data will be
     delivered to the destination.

     Because it makes so few guarantees, IP is a very simple protocol.
     This means that it can be implemented fairly easily and can run on
     systems that have modest processing power and small amounts of
     memory. It also means that IP demands only minimal functionality
     from the underlying medium (the physical network that carries
     packets on behalf of IP) and can be deployed on a wide variety of
     networking technologies.

     The no-promises type of service offered by IP is not directly
     useful to many applications. Applications usually depend on TCP or
     UDP to provide assurances of of data integrity and (in TCP's case)
     ordered and complete data delivery.

     The fundamentals of IP are defined in RFC 791. RFC 1122 summarises
     the requirements that must be met by an IP implementation in an
     Internet host, and RFC 1812 summarises the IP requirements for an
     Internet router.

  2. How Is IP Carried On A Network?

     IP really isn't very fussy about how its packets are transported.
     The details of how an IP packet is carried over a particular kind
     of network are usually chosen to be convenient for the network
     itself. As long as the transmitter and receiver observe some
     convention that allows IP packets to be differentiated from any
     other data that might be seen by the receiver, then IP can be used
     to carry data between those stations.

     On a LAN, IP is carried in the data portion of the LAN frame and
     the frame header contains additional information that identifies
     the frame an an IP frame. Different LAN's have different
     conventions for carrying that additional information. On an
     Ethernet the Ethertype field is used; a value of 0x0800 identifies
     a frame that contains IP data. FDDI and Token Ring use frames that
     conform to IEEE 802 Logical Link Control, and on those LAN's IP is
     carried in Unnumbered Information frames with Source and
     Destination LSAP's of 0xAA and a SNAP header of 00-00-00-08-00.

     The only thing that IP cares strongly about is the maximum size of
     a frame that can be carried on the medium. This controls whether,
     and to what extent, IP must break down large data packets into a
     train of smaller packets before arranging for them to be
     transmitted on the medium. This activity is called "fragmentation"
     and the resulting smaller and incomplete packets are called
     "fragments". The final destination is responsible for rebuilding
     the original IP packet from its fragments, an activity called
     "fragment reassembly".

  3. Does IP Protect Data On The Network?

     IP itself does not guarantee to deliver data correctly. It leaves
     all issues of data protection to the transport protocol. Both TCP
     and UDP have mechanisms that guarantee that the data they deliver
     to an application is correct.

     IP does try to protect the packet's IP header, the relatively
     small part of each packet that controls how the packet is moved
     through the network. It does this by calculating a checksum on the
     header fields and including that checksum in the transmitted
     packet. The receiver verifies the IP header checksum before
     processing the packet. Packets whose checksums no longer match
     have been damaged in some way and are simply discarded.

     The IP checksum is discussed in detail in RFC 1071, which also
     includes sample code for calculating the checksum. RFC 1141 and
     RFC 1624 describe incremental modification of an existing
     checksum, which can be useful in machines such as routers which
     modify fields in the IP header while forwarding a packet and
     therefore need to compute a new header checksum.

     The same checksum algorithm is used by TCP and UDP, although they
     include the data portion of the packet (not just the header) in
     their calculations.

  4. What is ARP?

     Address Resolution Protocol (ARP) is a mechanism that can be used
     by IP to find the link-layer station address that corresponds to a
     particular IP address. It defines a method that is used to ask,
     and answer, the question "what MAC address corresponds to a given
     IP address?". ARP sends broadcast frames to obtain this
     information dynamically, so it can only be used on media that
     support broadcast frames. Most LAN's (including Ethernet, FDDI,
     and Token Ring) have a broadcast capability and ARP is used when
     IP is running on those media. ARP is defined in RFC 826. That
     definition assumes an Ethernet LAN. Additional details for ARP on
     networks that use IEEE 802.2 frame formats (IEEE 802.3 CSMA/CD,
     IEEE 802.4, IEEE 802.5 Token Ring) are in RFC 1042. ARP on FDDI is
     described in RFC 1390.

     When IP is runnning over non-broadcast media (say, X.25 or ATM)
     some other mechanism is used to match IP addresses to media
     addresses. IP really doesn't care how the media address is

     RFC 903 defines Reverse ARP (RARP) which lets a station ask the
     question "which IP address corresponds to a given MAC address?".
     RARP is typically used to let a piece of diskless equipment
     discover its own IP address as part of its boot procedure. RARP is
     rarely used by modern equipment; it has been supplanted by the
     Boot Protocol (BOOTP) defined in RFC 1542. BOOTP in turn is being
     supplanted by the Dynamic Host Configuration Protocol (DHCP).

  5. What is IPv6?

     IP Version 6 (IPv6) is the newest version of IP, sometimes called
     IPng for "IP, Next Generation". IPv6 is fairly well defined but is
     not yet widely deployed. The main differences between IPv6 and the
     current widely-deployed version of IP (which is IPv4) are:

	o IPv6 uses larger addresses (128 bits instead of 32 bits in
	  IPv4) and so can support many more devices on the network,

	o IPv6 includes features like authentication and multicasting
	  that had been bolted on to IPv4 in a piecemeal fashion over
	  the years.

     Information on IPv6 can be found on the IPv6 home page at

  6. What happened to IPv5?

     Or, ""Why are we skipping from IPv4 to IPv6?"

     IPv5 never existed. The version number "5" in the IP header was
     assigned to identify packets carrying an experimental non-IP
     real-time stream protocol called ST. ST was never widely used, but
     since the version number 5 had already been allocated the new
     version of IP was given its own unique identifying number, 6. ST
     is described in RFC 1819.

  7. What is the 6bone?

     The 6bone is the experimental IPv6 backbone being developed using
     IPv6-in-IPv4 tunnels. This is intended for early experimentation
     with IPv6 and is not a production service.

  8. What is the MBONE?

     The Multicast backBONE (MBONE) is a multicast-capable portion of
     the Internet backbone. Multicast support over IP is provided by a
     protocol called IGMP (Internet Group Management Protocol) which is
     defined in RFC 1112. The MBONE is still a research prototype, but
     it extends through most of the core of the Internet (including
     North America, Europe, and Australia). It is typically used to
     relay multimedia (audio and low bandwidth video) presentations
     from a single source to multiple receiving sites dispersed over
     the Internet.

     A slightly dated MBONE FAQ is available by anonymous FTP from

  9. What is IPsec?

     IPsec stands for "IP Security". The IPsec working group of the
     IETF is developing standards for cryptographic authentication and
     for encryption within IP. The base specifications are defined in
     RFC's 1825, 1826 and 1827. Products that implement these are
     beginning to appear.

     A freely distributable implementation of IPsec for IPv4 and IPsec
     for IPv6 is included in the NRL IPv6/IPsec distribution for
     4.4-Lite BSD.  The NRL software is available from
     <> (for distribution within the
     US only), from
     <> (for
     distribution within the US and Canada), and from
     <> (for unrestricted distribution).

     (Some countries consider encryption software to have military
     significance and so restrict the export and import of such
     software, which is why there are geographical restrictions on the
     areas served by the above sites.)

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About TCP

  1. What is TCP?

     Transmission Control Protocol (TCP) provides a reliable
     byte-stream transfer service between two endpoints on an internet.
     TCP depends on IP to move packets around the network on its
     behalf. IP is inherently unreliable, so TCP protects against data
     loss, data corruption, packet reordering and data duplication by
     adding checksums and sequence numbers to transmitted data and, on
     the receiving side, sending back packets that acknowledge the
     receipt of data.

     Before sending data across the network, TCP establishes a
     connection with the destination via an exchange of management
     packets. The connection is destroyed, again via an exchange of
     management packets, when the application that was using TCP
     indicates that no more data will be transferred. In OSI terms, TCP
     is a Connection-Oriented Acknowledged Transport protocol.

     TCP has a multi-stage flow-control mechanism which continuously
     adjusts the sender's data rate in an attempt to achieve maximum
     data throughput while avoiding congestion and subsequent packet
     losses in the network.  It also attempts to make the best use of
     network resources by packing as much data as possible into a
     single IP packet, although this behaviour can be overridden by
     applications that demand immediate data transfer and don't care
     about the inefficiencies of small network packets.

     The fundamentals of TCP are defined in RFC 793, and later RFC's
     refine the protocol. RFC 1122 catalogues these refinements as of
     October 1989 and summarises the requirements that a TCP
     implementation must meet.

     TCP is still being developed. For instance, RFC 1323 introduces a
     TCP option that can be useful when traffic is being carried over
     high-capacity links. It is important that such developments are
     backwards-compatible. That is, a TCP implementation that supports
     a new feature must continue to work with older TCP implementations
     that do not support that feature.

  2. How does TCP try to avoid network meltdown?

     TCP includes several mechanisms that attempt to sustain good data
     transfer rates while avoiding placing excessive load on the
     network.  TCP's "Slow Start", "Congestion Avoidance", "Fast
     Retransmit" and "Fast Recovery" algorithms are summarised in RFC
     2001. TCP also mandates an algorithm that avoids "Silly Window
     Syndrome" (SWS), an undesirable condition that results in very
     small chunks of data being transferred between sender and
     receiver. SWS Avoidance is discussed in RFC 813. The "Nagle
     Algorithm", which prevents the sending side of TCP from flooding
     the network with a train of small frames, is described in RFC

     Van Jacobson has done significant work on this aspect of TCP's
     behaviour. The FAQ used to contain a couple of pieces of
     historically interesting pieces of Van's email concerning an early
     implementation of congestion avoidance, but in the interests of
     saving space they've been removed and can instead be obtained by
     anonymous FTP from the end-to-end mailing list archive at
     <>. PostScript slides
     of a presentation on this implementation of congestion avoidance
     can be obtained by anonymous FTP from

     That directory contains several other interesting TCP-related
     papers, including one
     (<>) by Sally Floyd that
     discusses a algorithm that attempts to give TCP the ability to
     recover quickly from packet loss in a network.

  3. How do applications coexist over TCP and UDP?

     Each application running over TCP or UDP distinguishes itself from
     other applications using the service by reserving and using a
     16-bit port number. Destination and source port numbers are placed
     in the UDP and TCP headers by the originator of the packet before
     it is given to IP, and the destination port number allows the
     packet to be delivered to the intended recipient at the
     destination system.

     So, a system may have a Telnet server listening for packets on TCP
     port 23 while an FTP server listens for packets on TCP port 21 and
     a DNS server listens for packets on port 53. TCP examines the port
     number in each received frame and uses it to figure out which
     server gets the data. UDP has its own similar set of port

     Many servers, like the ones in this example, always listen on the
     same well-known port number. The actual port number is arbitrary,
     but is fixed by tradition and by an official allocation or
     "assignment" of the number by the Internet Assigned Numbers
     Authority (IANA).

  4. Where do I find assigned port numbers?

     The IANA allocates and keeps track of all kinds of arbitrary
     numbers used by TCP/IP, including well-known port numbers. The
     entire collection is published periodically in an RFC called the
     Assigned Numbers RFC, each of which supersedes the previous one in
     the series.  The current Assigned Numbers RFC is RFC 1700.

     The Assigned Numbers document can also be obtained directly by FTP
     from the IANA at <>.

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About UDP

  1. What is UDP?

     User Datagram Protocol (UDP) provides an unreliable packetized
     data transfer service between endpoints on an internet. UDP
     depends on IP to move packets around the network on its behalf.

     UDP does not guarantee to actually deliver the data to the
     destination, nor does it guarantee that data packets will be
     delivered to the destination in the order in which they were sent
     by the source, nor does it guarantee that only one copy of the
     data will be delivered to the destination. UDP does guarantee data
     integrity, and it does this by adding a checksum to the data
     before transmission. (Some machines run with UDP checksum
     generation disabled, in which case data corruption or truncation
     can go undetected. Very few people think this is a good idea.)

     The fundamentals of UDP are defined in RFC 768. RFC 1122
     summarises the requirements that a UDP implementation must meet.

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About ICMP

  1. What is ICMP?

     Internet Control Message Protocol (ICMP) defines a small number of
     messages used for diagnostic and management purposes. ICMP depends
     on IP to move packets around the network on its behalf.

     The fundamentals of ICMP are defined in RFC 792. RFC 1122
     summarises the requirements that must be met by an ICMP
     implementation in an Internet host, and RFC 1812 summarises the
     ICMP requirements for an Internet router.

     ICMP is basically IP's internal network management protocol and is
     not intended for use by applications. Two well known exceptions
     are the ping and traceroute diagnostic utilities:

	o ping sends and receives ICMP "ECHO" packets, where the
	  response packet can be taken as evidence that the target host
	  is at least minimally active on the network, and

	o traceroute sends UDP packets and infers the route taken to
	  the target from ICMP "TIME-TO-LIVE EXCEEDED" or "PORT
	  UNREACHABLE" packets returned by the network. (Microsoft's
	  TRACERT sends ICMP "ECHO" packets rather than UDP packets,
	  and so receives ICMP "TIME-TO-LIVE EXCEEDED" or "ECHO
	  RESPONSE" packets in return.)

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TCP/IP Network Operations

  1. How can I measure the performance of an IP link?

     You can get a quick approximation by timing how long it takes to
     FTP or RCP a large file over the link, but bear in mind that that
     measurement will be skewed by the time spent in dealing with the
     local and remote filesystems, not simply with the network itself.
     And remember to measure the time it takes to receive a file, not
     the time it takes to send it; the sender can report completion
     even though large amounts of data are still buffered locally by
     TCP and have not yet been delivered to the destination.

     Two well-known open-source programs that measure and report
     throughput over an IP link without involving the filesystem are:

	o TTCP, available for anonymous ftp from the Silicon Graphics
	  FTP archive at <>.

	o Rick Jones' NETPERF, available on the Web at

     If neither of those tools does what you want then you might find
     something that meets your needs in CAIDA's measurement tools list
     at <>.

  2. What IP addresses should I assign to machines on a private

     You shouldn't use IP addresses that have been assigned to some
     other organisation, because if knowledge of your network ever gets
     leaked onto the Internet they may disrupt that innocent
     organisation's activity. RFC 1918 provides a solution for this
     problem by allocating several IP address ranges specifically for
     use on private networks.  These addresses will never be assigned
     to any organisation and are never supposed to appear on the
     Internet. The ranges are:

             Class A:    through
             Class B:  through
             Class C: through

  3. Can I set up a gateway to the Internet that translates IP
     addresses, so that I don't have to change all our internal
     addresses to an official network?

     This is called Network Address Translation, or NAT. In general it
     is a difficult thing to do properly because many applications
     embed IP addresses in the application-level data (FTP's "PORT"
     command is a notable example) so NAT isn't simply a matter of
     translating addresses in the IP header and recalculating header
     checksums. Also, if the network number(s) you're using match those
     assigned to another organisation, your gateway may not be able to
     communicate with that organisation. As noted above, RFC 1918
     proposes network numbers that are reserved for private use, to
     avoid such conflicts, but if you're already using a different
     network number this won't help you.

     However, there are several products that do attempt to provide
     this kind of on-the-fly NAT. Linux provides NAT through its "IP
     Masquerading" feature, and many firewall and router vendors offer
     NAT capabilities in their products -- look for "Network Address
     Translation" in your favourite Web search engine to get a list of
     vendors. Proxy servers developed for firewalls can also sometimes
     be used as a substitute for an address-translating gateway for
     particular application protocols. This is discussed in more detail
     in the FAQ for the newsgroup. That FAQ can
     be viewed on the Web at <>.

  4. Can I use a single bit subnet?

     The answer used to be a straightforward "no", because a 1-bit
     subnet can only have a subnet part of all-ones or all-zeroes, both
     of which were assigned special meanings when the subnetting
     concept was originally defined. (All-ones meant "broadcast, all
     subnets of this net" and all-zeroes meant "this subnet, regardless
     of its actual subnet number".)

     However, the old definition of subnetting has been superseded by
     the concept of Classless Inter-Domain Routing (CIDR, pronounced
     'cider').  Under CIDR the subnet doesn't really have an existence
     of its own and the subnet mask simply provides the mechanism for
     isolating an arbitrarily-sized network portion of an IP address
     from the remaining host part. As CIDR-aware equipment is deployed
     it becomes increasingly like that you will be able to use a 1-bit
     subnet with at least some particular combinations of networking
     equipment. However, it's still not safe to assume that a 1-bit
     subnet will work properly with all kinds of equipment.

     As Steinar Haug explains:

          From RFC 1122:

          > 3.3.6  Broadcasts
          > Section defined the four standard IP broadcast address
          > forms:
          >   Limited Broadcast:              {-1, -1}
          >   Directed Broadcast:             {<Network-number>, -1}
          >   Subnet Directed Broadcast:      {<Network-number>, <Subnet-number>, -1}
          >   All-Subnets Directed Broadcast: {<Network-number>, -1, -1}

          All-Subnets Directed broadcasts are being deprecated in favor of IP
          multicast, but were very much defined at the time RFC1122 was written.
          Thus a Subnet Directed Broadcast to a subnet of all ones is not
          distinguishable from an All-Subnets Directed Broadcast.

          For those old systems that used all zeros for broadcast in IP
          addresses, a similar argument can be made against the subnet of all

          Also, for old routing protocols like RIP, a route to subnet zero is not
          distinguishable from the route to the entire network number (except
          possibly by context).

          Most of today's systems don't support variable length subnet masks
          (VLSM), and for such systems the above is true. However, all the major
          router vendors and *some* Unix systems (BSD 4.4 based ones) support
          VLSMs, and in that case the situation is more complicated :-)

          With VLSMs (necessary to support CIDR, see RFC 1519), you can utilize
          the address space more efficiently. Routing lookups are based on
          *longest* match, and this means that you can for instance subnet the
          class C net with a mask of (27 bits) in addition to the
          subnet mask of (26 bits) given above. You will then be
          able to use the addresses x.x.x.33 through x.x.x.62 (first three bits
          001) and the addresses x.x.x.193 through x.x.x.222 (first three bits
          110) with this new subnet mask. And you can continue with a subnet mask
          of 28 bits, etc.  (Note also, by the way, that non-contiguous subnet
          masks are deprecated.)

          This is all very nicely covered in the paper by Havard Eidnes:

               Practical Considerations for Network Address using a
               CIDR Block Allocation
               Proceedings of INET '93

          This paper is available with anonymous FTP from

          The same paper, with minor revisions, is one of the articles in the
          special Internetworking issue of Communications of the ACM (last month,
          I believe).

          Steinar Haug, SINTEF RUNIT, University of Trondheim, NORWAY

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TCP/IP Protocol Implementations

  1. Where can I find TCP/IP source code?

     Code used in the venerable Net-2 version of Berkeley Unix is
     available by anonymous FTP from
     <> (at UUNet
     in Virginia, US) and
     <> (at Compaq in
     California, US).

     Source code for the TCP/IP implementations in the current dialects
     of BSD Unix is available. Instructions on how to access the
     sources through FTP and other methods is detailed on their
     respective websites:  FreeBSD at <>; NetBSD
     at <>; and OpenBSD at

     Source for the Unix-like Linux operating system is at

     Source for the TCP/IP implementation of the Xinu operating system
     discussed in Comer & D. L. Stevens' "Internetworking with TCP/IP
     Volume II" is at <>.

     WATTCP is a DOS TCP/IP stack derived from the NCSA Telnet program
     and much enhanced. It is available from many DOS software archive
     sites as WATTCP.ZIP. This file includes some example programs and
     complete source code. The interface isn't BSD sockets but is well
     suited to PC type work.

     KA9Q is Phil Karn's network operating system for PC's. It includes
     a TCP/IP implementation originally developed for use over packet
     radio.  Source is available from Phil's website at

  2. Where can I find TCP/IP application source code?

     Source code for the various TCP/IP applications delivered with the
     current BSD Unix flavours is available through the FreeBSD, NetBSD
     and OpenBSD websites noted in the previous section.

     Linux application source is at <>.
     Much of the application source used by Linux was originally
     developed by the GNU Project whose website is at

     Code from Comer & D. L. Stevens' "Internetworking with TCP/IP
     Volume III" is available by anonymous FTP from

     Code from W. R. Stevens' "TCP/IP Illustrated, Volume 1" is
     available from

     Source code for some well-known cross-system TCP/IP applications
     (BIND, sendmail, Apache and so on) is available from the various
     organisations that sponsor the applications. See Part 2 of the FAQ
     for details.

  3. Where can I find IPv6 source code?

     There are several freely distributable implementations of IPv6,
     particularly for BSD and Linux. You can find detailed information at
     part of the IPv6 home site mentioned above.

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Further Sources of Information

  1. TCP/IP-related newsgroups and FAQ lists

     Collections of newsgroup FAQ documents are archived at many
     locations including <> and
     <>.  The following newsgroups are
     particularly relevant to TCP/IP:

          comp.protocols.tcp-ip covers all of the TCP/IP suite.

	  comp.protocols.dns.bind covers the BIND suite, which contains
	  server and client implementations of DNS. discusses DNS global
	  administration and politics.

	  comp.protocols.nfs covers NFS protocol, implementation, and

	  comp.protocols.snmp covers SNMP definition, implementation,
	  and administration.

	  comp.protocols.time.ntp covers NTP definition,
	  implementation, and administration.

	  comp.protocols.tcp-ip.ibmpc discusses TCP/IP for IBM(-like)
	  personal computers. The group's FAQ is available at
	  <>. discusses TCP/IP on
	  Microsoft Windows machines. covers the
	  Winsock sockets API on Microsoft Windows machines. The
	  group's FAQ is available at

          comp.os.os2.networking.tcp-ip discusses TCP/IP on OS/2.

	  comp.dcom.lans.ethernet covers Ethernet and IEEE 802.3 LAN's.
	  The group's FAQ is available at

          comp.dcom.lans.fddi covers FDDI LAN's.

	  comp.dcom.lans.token-ring covers IBM Token Ring and IEEE
	  802.5 LAN's.

	  comp.dcom.lans.misc covers all other types of LAN.

	  comp.protocols.ppp covers PPP and SLIP. The group's FAQ is
	  available at <>. discusses cisco products.

	  comp.dcom.sys.wellfleet discusses Wellfleet (now Bay
	  Networks) products.

  2. Are there any good books on TCP/IP?

     Yes, lots of them, far too many to list here. Uri Raz maintains a
     TCP/IP bibliography (the "TCP/IP Resources List") that is posted
     to the comp.protocols.tcp-ip newsgroup on a monthly basis. It is
     available on the Web at
     <> and
     or can be retrieved by anonymous FTP from

     However, a couple of books that always head the list of
     recommended reading are:

	  "Internetworking with TCP/IP Volume I (Principles, Protocols,
	  and Architecture)" by Douglas E. Comer, published by Prentice
	  Hall, 1991 (ISBN 0-13-468505-9). This is an introductory book
	  which covers all of the fundamental protocols, including IP,
	  UDP, TCP, and the gateway protocols. It also discusses some
	  higher level protocols such as FTP, Telnet, and NFS.

	  "TCP/IP Illustrated, Volume 1: The Protocols" by W. Richard
	  Stevens, published by Addison-Wesley, 1994 (ISBN
	  0-201-63346-9).  This book explains the TCP/IP protocols and
	  several application protocols in exquisite detail. It
	  contains many real-life traces of the protocols in action,
	  which is especially valuable for people who need to
	  understand the protocols in depth.

     If you're writing programs that use TCP/IP then the classic text
     is "Unix Network Programming" by W. Richard Stevens, published by
     Prentice Hall, 1990 (ISBN 0-13-949876-1). It's now being rewritten
     as a three volume set. The first volume "Unix Network Programming:
     Networking APIs: Sockets and Xti" published by Prentice Hall, 1997
     (ISBN 013490012X), contains just about everything you need to know
     about using TCP/IP and includes material on topics (eg IPv6,
     multicasting, threads) that are not covered in the original UNP.

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This compilation contains the opinions of the FAQ maintainer and the
various FAQ contributors. Any resemblance to the opinions of the FAQ
maintainer's employer is entirely coincidental.

Copyright (C) Mike Oliver 1997-1999. All Rights Reserved.

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