Subnetwork addressing scheme
RFC - Unknown
(January 1985; No errata)
||RFC Editor Note
RFC 932 (Unknown)
||Send notices to
Network Working Group David D. Clark
Request for Comments: 932 MIT, LCS
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.
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
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
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 [Page 1]
RFC 932 January 1985
A Subnetwork Addressing Scheme
The gateways and hosts attached to this subnetted network use these
addresses as ordinary Class C addresses. Thus, no modification to
any host software is required for hosts attached to a subnetwork.
For gateways not directly attached to the subnetted network, it is an
unacceptable burden to separately store the routing information to
each of the subnets. The goal of any subnet addressing scheme is to
provide a strategy by which distant gateways can store routing
information for the network as a whole. In this scheme, since the
first two bytes of the address is the same for every subnet in the
network, those first two bytes can be stored and manipulated as if
they are a single Class B address by a distant gateway. These
addresses, which can be used either as a Class B or Class C address
as appropriate, have been informally called Class "B 1/2" addresses.
In more detail, a gateway would treat Class C addresses as follows
under the scheme. First, test to see whether the high order bit of
the address is on. If not, the address is an ordinary Class C
address and should be treated as such.
If the bit is on, this Class C address identifies a subnet of a
network. Test to see if this gateway is attached to that network.
If so, treat the address as an ordinary Class C address.
If the gateway is not attached to the network containing that
subnetwork, discard the third byte of the Class C address and treat
the resulting two bytes as a Class B address. Note that there can be
no conflict between this two-byte pattern and an ordinary Class B
address, because the first bits of this address are not those of a
valid Class B address, but rather those of a Class C address.
If a network grows to more than 256 subnetworks, it will be necessary
to design two distinct blocks of special Class C addresses, and to
view this aggregate as two separate networks. However, the gateways
of these two networks can, by proper design, run a joint routing
algorithm which maintains optimal routes between the two halves, even
if they are connected together by a number of gateways.
Indeed, in general it is possible for gateways that are not directly
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