RFC 932 (rfc932) - Page 1 of 4
Subnetwork addressing scheme
Alternative Format: Original Text Document
Network Working Group David D. Clark
Request for Comments: 932 MIT, LCS
January 1985
A SUBNETWORK 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
Several recent RFCs have discussed the need for a "subnet" structure
within the internet addressing scheme, and have proposed strategies
for "subnetwork" addressing and routing. In particular, Jeff Mogul
in his RFC-917, "Internet Subnets", describes an addressing scheme in
which a variable number of the leading bits of the host portion of
the address are used to identify the subnet. The drawback to this
scheme is that it is necessary to modify the host implementation in
order to implement it. While the modification is a simple one, it is
necessary to retrofit it into all implementations, including those
which are already in the field. (See RFC-917 by Mogul for various
alternative approaches to this problem, such as using Address
Resolution Protocol.)
This RFC proposes an alternative addressing scheme for subnets which,
in most cases, requires no modification to host software whatsoever.
The drawbacks of this scheme are that the total number of subnets in
any one network are limited, and that modification is required to all
gateways.
THE PROPOSAL
In this scheme, the individual subnets of a network are numbered
using Class C addresses. Since it is necessary with this scheme that
a Class C address used to number a subnet be distinguishable from a
Class C address used to number an isolated network, we will reserve
for subnetworks the upper half of the Class C address space, in other
words all those Class C addresses for which the high order bit is on.
When a network is to be organized as a series of subnetworks, a block
of these reserved Class C addresses will be assigned to that network,
specifically a block of 256 addresses having the two first bytes
identical. Thus, the various subnetworks of a network are
distinguished by the third byte of the Internet address. (This
addressing scheme implies the limitation that there can only be 256
subnetworks in a net. If more networks are required, two blocks will
have to be allocated, and the total viewed as two separate networks.)
Clark
