Address mappings
How early Internet address fields mapped to the local addresses of specific networks.
Postel, J., "Address mappings", RFC 796, DOI 10.17487/RFC0796, September 1981, <https://www.rfc-editor.org/info/rfc796>.
@misc{rfc796,
series = {Request for Comments},
number = 796,
howpublished = {RFC 796},
publisher = {RFC Editor},
doi = {10.17487/RFC0796},
url = {https://www.rfc-editor.org/info/rfc796},
author = {J. Postel},
title = {{Address mappings}},
pagetotal = 7,
year = 1981,
month = sep,
} RFC text
Contents
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
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
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
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
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
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
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.
ISO Plain Language
On this page
§1Internet Address Classes
An Internet address is a 32-bit quantity with a network number and a local address. The local address identifies a host within the network named by the network number.
Class A addresses use a 7-bit network number and a 24-bit local address. Class B uses a 14-bit network number and a 16-bit local address. Class C uses a 21-bit network number and an 8-bit local address.
An Internet address has a part for the network and a part for a host on that network. This memo describes three layouts for dividing those parts.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
CLASS-BIT | Class bit | 1 bits | 0 | Leading zero marks a class A address. |
NETWORK | Network | 7 bits | Network number. | |
LOCAL | Local Address | 3 bytes | Address within the network. |
§2Mapping Local Network Addresses
The memo shows how each network's own address fits into the local part of an Internet address. The ARPANET has network number 10 and uses its 24-bit host, logical-host and IMP fields directly; its logical-host field is set to zero because the IMPs did not yet support that addressing form.
Other examples include AUTODIN II (network 26), the Distributed Computing Networks at COMSAT and UCL (networks 29 and 30), the Experimental Data Network (21), LCSNET (18), several packet radio networks, SATNET (4) and WBCNET (28). The diagrams show which address bits each network uses.
Each network places its own address fields inside the local part of the Internet address. The memo gives separate layouts for the networks it describes.
§3Different Local Address Fields
The address mappings vary with each network's address format. AUTODIN II and several packet radio networks use 16-bit host or subscriber addresses. The DCNs divide 16 bits into an 8-bit host identifier and an 8-bit process identifier.
SATNET addresses can be assigned independently of physical location, and a physical host may have several logical addresses. WBCNET divides its 16-bit address into a HAP number and a local address. These examples show why a network-specific mapping is needed.
The network-specific fields can identify hosts, processes or logical addresses. The RFC explains each layout rather than defining one format for all networks.
§4References and Scope
This memo is a set of address mapping descriptions, not a procedure for assigning addresses. It refers to RFC 791 for the Internet Protocol specification and cites separate documents for additional WBCNET and LCSNET address details.
The document describes the mappings shown in its diagrams. It directs readers to other documents for additional details about some networks.