J. Postel
IEN 115 ISI
August 1979
ADDRESS MAPPINGS
----------------
Internet Address
----------------
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, divided into an 8 bit
network number and a 24 bit local address as shown below.
+--------+--------+--------+--------+
| Net | Local Address |
+--------+--------+--------+--------+
The local address carries information to address a host in the
network identified by the network number. Since each network has a
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:
ARPANET
-------
The ARPANET (with 96 bit leaders) has 24 bit addresses. The 24
bits are assigned to host, logical host, and IMP fields as
illustrated below. These 24 bit addresses are used directly
for the 24 bit local address of the internet address.
The network number of the ARPANET is 10.
+--------+--------+--------+
| HOST | LH | IMP |
+--------+--------+--------+
8 8 8
Postel [Page 1]
August 1979 IEN 115 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. +--------+----------------+ | ZERO | HOST | +--------+----------------+ 8 16 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 SF-PR-1 2 SILL-PR 5 SF-PR-2 6 BRAGG-PR 9 DC-PR 20 +--------+----------------+ | ZERO | HOST | +--------+----------------+ 8 16 LCSNET ------ The LCS NET at MIT's Laboratory for Computer Science uses 32 bit addresses of several formats. Please see [2] 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. Postel [Page 2]
August 1979 IEN 115 Address Mappings The network number of the LCS NET is 18. +--------+--------+--------+ | SUB |RESERVED| HOST | | NET | | | +--------+--------+--------+ 8 8 8 EDN --- The Experimental Data Network at the Defense Communication Engineering Center (DCEC) uses the old 1822 32 bit leader, and so has 8 bit addresses. These 8 bit EDN addresses are located in the 24 bit internet local address as shown below. The network number of the EDN is 21. +------+--+----------+------+ | | H| | | | ZERO | O| ZERO | IMP | | | S| | | | | T| | | +------+--+----------+------+ 6 2 10 6 DCNs ---- The 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 CONSAT-DCN is 29, and of the UCL-DCN is 30. +--------+--------+--------+ | ZERO | HID | PID | +--------+--------+--------+ 8 8 8 Postel [Page 3]
August 1979
IEN 115 Address Mappings
References
----------
[1] Jonathan B. Postel, "Internet Protocol," USC/Information
Sciences Institute, IEN 111, August 1979.
[2] 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 4]
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