RFC 936 (rfc936) - Page 1 of 4
Another Internet subnet addressing scheme
Alternative Format: Original Text Document
Network Working Group Michael J. Karels
Request for Comments: 936 UC Berkeley
February 1985
Another Internet Subnet Addressing Scheme
Status of this Memo
This RFC suggests a proposed protocol for the ARPA-Internet
community, and requests discussion and suggestions for improvements.
Distribution of this memo is unlimited.
Introduction
There have been several proposals for schemes to allow the use of a
single Internet network number to refer to a collection of physical
networks under common administration which are reachable from the
rest of the Internet by a common route. Such schemes allow a
simplified view of an otherwise complicated topology from hosts and
gateways outside of this collection. They allow the complexity of
the number and type of these networks, and routing to them, to be
localized. Additions and changes in configuration thus cause no
detectable change, and no interruption of service, due to slow
propagation of routing and other information outside of the local
environment. These schemes also simplify the administration of the
network, as changes do not require allocation of new network numbers
for each new cable installed. The motivation for explicit or
implicit subnets, several of the alternatives, and descriptions of
existing implementations of this type have been described in detail
[1,2]. This proposal discusses an alternative scheme, one that has
been in use at the University of California, Berkeley since
April 1984.
Subnet Addressing at Berkeley
As in the proposal by Jeff Mogul in RFC-917, the Berkeley subnet
addressing utilizes encoding of the host part of the Internet
address. Hosts and gateways on the local network are able to
determine the subnet number from each local address, and then route
local packets based on the subnet number. Logically, the collection
of subnets appears to external sites to be a single, homogenous
network. Internally, however, each subnet is distinguished from the
others and from other networks, and internal routing decisions are
based on the subnet rather than the network number.
The encoding of subnet addresses is similar to that proposed in
RFC-917. In decomposing an Internet address into the network and
host parts, the algorithm is modified if the network is "local", that
is, if the network is a directly-connected network under local
administrative control. (Networks are marked as local or non-local
Karels
