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IETF RFC 796
Address mappings
Last modified on Thursday, October 15th, 1992
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Network Working Group J. Postel
Request for Comments: 796 ISI
Replaces: IEN 115 September 1981
ADDRESS MAPPINGS
----------------
Internet Addresses
------------------
This memo describes the relationship between address fields used in
the Internet Protocol (IP) [1] and several specific networks.
An internet address is a 32 bit quantity, with several codings as
shown below.
The first type (or class a) of address has a 7-bit network number and
a 24-bit local address.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| NETWORK | Local Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Class A Address
The second type (or class b) of address has a 14-bit network number
and a 16-bit local address.
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 0| NETWORK | Local Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Class B Address
The third type (or class c) of address has a 21-bit network number
and a 8-bit local address.
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 1 0| NETWORK | Local Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Class C Address
The local address carries information to address a host in the
network identified by the network number. Since each network has a
Postel PAGE 1 
September 1981
RFC 796 Address Mappings
particular address format and length, the following section describes
the mapping between internet local addresses and the actual address
format used in the particular network.
Internet to Local Net Address Mappings
--------------------------------------
The following transformations are used to convert internet addresses
to local net addresses and vice versa:
AUTODIN II
----------
The AUTODIN II has 16 bit subscriber addresses which identify
either a host or a terminal. These addresses may be assigned
independent of location. The 16 bit AUTODIN II address is
located in the 24 bit internet local address as shown below.
The network number of the AUTODIN II is 26 (Class A).
+----------------+
| HOST/TERMINAL | AUTODIN II
+----------------+
16
+--------+--------+--------+--------+
| 26 | ZERO | HOST/TERMINAL | IP
+--------+--------+--------+--------+
8 8 16
Postel PAGE 2
September 1981
RFC 796 Address Mappings
ARPANET
-------
The ARPANET (with 96 bit leaders) has 24 bit addresses. The 24
bits are assigned to host, logical host, and IMP leader fields
as illustrated below. These 24 bit addresses are used directly
for the 24 bit local address of the internet address. However,
the ARPANET IMPs do not yet support this form of logical
addressing so the logical host field is set to zero in the
leader.
The network number of the ARPANET is 10 (Class A).
+--------+--------+--------+
| HOST | ZERO | IMP | ARPANET
+--------+--------+--------+
8 8 8
+--------+--------+--------+--------+
| 10 | HOST | LH | IMP | IP
+--------+--------+--------+--------+
8 8 8 8
DCNs
----
The Distributed Computing Networks (DCNs) at COMSAT and UCL use
16 bit addresses divided into an 8 bit host identifier (HID),
and an 8 bit process identifier (PID). The format locates
these 16 bits in the low order 16 bits of the 24 bit internet
address, as shown below.
The network number of the COMSAT-DCN is 29 (Class A), and of
the UCL-DCN is 30 (Class A).
+--------+--------+
| HID | PID | DCN
+--------+--------+
8 8
+--------+--------+--------+--------+
| 18 | ZERO | HID | PID | IP
+--------+--------+--------+--------+
8 8 8 8
Postel PAGE 3
September 1981
RFC 796 Address Mappings
EDN
---
The Experimental Data Network at the Defense Communication
Engineering Center (DCEC) uses the same type of addresses as
the ARPANET (with 96 bit leaders) and has 24 bit addresses.
The 24 bits are assigned to host, logical host, and IMP leader
fields as illustrated below. These 24 bit addresses are used
directly for the 24 bit local address of the internet address.
However, the IMPs do not yet support this form of logical
addressing so the logical host field is set to zero in the
leader.
The network number of the EDN is 21 (Class A).
+--------+--------+--------+
| HOST | ZERO | IMP | EDN
+--------+--------+--------+
8 8 8
+--------+--------+--------+--------+
| 21 | HOST | LH | IMP | IP
+--------+--------+--------+--------+
8 8 8 8
LCSNET
------
The LCS NET at MIT's Laboratory for Computer Science uses 32
bit addresses of several formats. Please see [3] for more
details. The most common format locates the low order 24 bits
of the 32 bit LCS NET address in the 24 bit internet local
address, as shown below.
The network number of the LCS NET is 18 (Class A).
+--------+--------+--------+
| SUBNET |RESERVED| HOST | LCSNET
+--------+--------+--------+
8 8 8
+--------+--------+--------+--------+
| 18 | SUBNET |RESERVED| HOST | IP
+--------+--------+--------+--------+
8 8 8 8
Postel PAGE 4
September 1981
RFC 796 Address Mappings
PRNET
-----
The Packet Radio networks use 16 bit addresses. These are
independent of location (indeed the hosts may be mobile). The
16 bit PRNET addresses are located in the 24 bit internet local
address as shown below.
The network numbers of the PRNETs are:
BBN-PR 1 (Class A)
SF-PR-1 2 (Class A)
SILL-PR 5 (Class A)
SF-PR-2 6 (Class A)
BRAGG-PR 9 (Class A)
DC-PR 20 (Class A)
+--------+--------+
| HOST | PRNET
+--------+--------+
16
+--------+--------+--------+--------+
| net | ZERO | HOST | IP
+--------+--------+--------+--------+
8 8 16
Postel PAGE 5
September 1981
RFC 796 Address Mappings
SATNET
------
The Atlantic Satellite Packet Network has 16 bit addresses for
hosts. These addresses may be assigned independent of location
(i.e., ground station). It is also possible to assign several
addresses to one physical host, so the addresses are logical
addresses. The 16 bit SATNET address is located in the 24 bit
internet local address as shown below.
The network number of the SATNET is 4 (Class A).
+--------+--------+
| HOST | SATNET
+--------+--------+
16
+--------+--------+--------+--------+
| 4 | ZERO | HOST | IP
+--------+--------+--------+--------+
8 8 16
WBCNET
------
The Wideband Communication Satellite Packet Network (WBCNET)
Host Access Protocol (HAP) has 16 bit addresses for hosts. It
is possible to assign several addresses to one physical host,
so the addresses are logical addresses. The 16 bit WBCNET
address is divided into a HAP Number field and a Local Address
field, and is located in the 24 bit internet local address as
shown below. Please see [2] for more details.
The network number of the WBCNET is 28 (Class A).
+--------+--------+
| HAP NUM| LCL ADD| WBCNET
+--------+--------+
8 8
+--------+--------+--------+--------+
| 28 | HAP NUM| ZERO | LCL ADD| IP
+--------+--------+--------+--------+
8 8 8 8
Postel PAGE 6
September 1981
RFC 796 Address Mappings
References
----------
[1] Postel, J. (ed.), "Internet Protocol - DARPA Internet Program
Protocol Specification," RFC 791, USC/Information Sciences
Institute, September 1981.
[2] Pershing J., "Addressing Revisited," Bolt Beranek and Newman
Inc., W Note 27, May 1981.
[3] Noel Chiappa, David Clark, David Reed, "LCS Net Address
Format," M.I.T. Laboratory for Computer Science Network
Implementation, Note No.5, IEN 82, February 1979.
Postel PAGE 7
Address mappings
RFC TOTAL SIZE: 11239 bytes
PUBLICATION DATE: Thursday, October 15th, 1992
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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