Connected: An Internet Encyclopedia

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The files in this directory document the currently assigned values for
several series of numbers used in network protocol implementations.

The Internet Assigned Numbers Authority (IANA) is the central
coordinator for the assignment of unique parameter values for Internet
protocols.  The IANA is chartered by the Internet Society (ISOC) and
the Federal Network Council (FNC) to act as the clearinghouse to
assign and coordinate the use of numerous Internet protocol

The Internet protocol suite, as defined by the Internet Engineering
Task Force (IETF) and its steering group (the IESG), contains numerous
parameters, such as internet addresses, domain names, autonomous
system numbers (used in some routing protocols), protocol numbers,
port numbers, management information base object identifiers,
including private enterprise numbers, and many others.

The common use of the Internet protocols by the Internet community
requires that the particular values used in these parameter fields be
assigned uniquely.  It is the task of the IANA to make those unique
assignments as requested and to maintain a registry of the currently
assigned values.

Requests for parameter assignments (protocols, ports, etc.) should be
sent to <>.

Requests for SNMP network management private enterprise number
assignments should be sent to <>.

The IANA is located at and operated by the Information Sciences
Institute (ISI) of the University of Southern California (USC).

If you are developing a protocol or application that will require the
use of a link, socket, port, protocol, etc., please contact the IANA
to receive a number assignment.

        Joyce K. Reynolds
        Internet Assigned Numbers Authority
        USC - Information Sciences Institute
        4676 Admiralty Way
        Marina del Rey, California  90292-6695

        Electronic mail: IANA@ISI.EDU
        Phone: +1 310-822-1511

Most of the protocols are documented in the RFC series of notes.  Some
of the items listed are undocumented.  Further information on
protocols can be found in the memo, "Internet Official Protocol
Standards" (STD 1).

Data Notations

The convention in the documentation of Internet Protocols is to
express numbers in decimal and to picture data in "big-endian" order
[COHEN].  That is, fields are described left to right, with the most
significant octet on the left and the least significant octet on the

The order of transmission of the header and data described in this
document is resolved to the octet level.  Whenever a diagram shows a
group of octets, the order of transmission of those octets is the
normal order in which they are read in English.  For example, in the
following diagram the octets are transmitted in the order they are

    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   |       1       |       2       |       3       |       4       |
   |       5       |       6       |       7       |       8       |
   |       9       |      10       |      11       |      12       |

                       Transmission Order of Bytes

Whenever an octet represents a numeric quantity the left most bit in the
diagram is the high order or most significant bit.  That is, the bit
labeled 0 is the most significant bit.  For example, the following
diagram represents the value 170 (decimal).

                          0 1 2 3 4 5 6 7 
                         |1 0 1 0 1 0 1 0|

                        Significance of Bits

Similarly, whenever a multi-octet field represents a numeric quantity
the left most bit of the whole field is the most significant bit.  When
a multi-octet quantity is transmitted the most significant octet is
transmitted first.

Special Addresses

There are five classes of IP addresses: Class A through Class E.  Of
these, Classes A, B, and C are used for unicast addresses, Class D is
used for multicast addresses, and Class E addresses are reserved for
future use.

With the advent of classless addressing [CIDR1, CIDR2], the
network-number part of an address may be of any length, and the whole
notion of address classes becomes less important.

There are certain special cases for IP addresses.  These special cases
can be concisely summarized using the earlier notation for an IP

      IP-address ::=  { <Network-number>, <Host-number> }


      IP-address ::=  { <Network-number>, <Subnet-number>,
                                                      <Host-number> }

if we also use the notation "-1" to mean the field contains all 1
bits.  Some common special cases are as follows:

      (a)   {0, 0}

         This host on this network.  Can only be used as a source
         address (see note later).

      (b)   {0, <Host-number>}

         Specified host on this network.  Can only be used as a
         source address.

      (c)   { -1, -1}

         Limited broadcast.  Can only be used as a destination
         address, and a datagram with this address must never be
         forwarded outside the (sub-)net of the source.

      (d)   {<Network-number>, -1}

         Directed broadcast to specified network.  Can only be used
         as a destination address.

      (e)   {<Network-number>, <Subnet-number>, -1}

         Directed broadcast to specified subnet.  Can only be used as
         a destination address.

      (f)   {<Network-number>, -1, -1}

         Directed broadcast to all subnets of specified subnetted
         network.  Can only be used as a destination address.

      (g)   {127, <any>}

         Internal host loopback address.  Should never appear outside
         a host.


[COHEN]   Cohen, D., "On Holy Wars and a Plea for Peace", IEEE Computer
          Magazine, October 1981.

[CIDR1]   Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless
          Inter-Domain Routing (CIDR): an Address Assignment and
          Aggregation Strategy", RFC 1519, September 1993.

[CIDR2]   Rekhter, Y., and T. Li, "An Architecture for IP Address
          Allocation with CIDR", RFC 1518, September 1993.


Connected: An Internet Encyclopedia