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Network Working Group GADS
Request for Comments: 940
April 1985

Toward an Internet Standard Scheme for Subnetting


STATUS OF THIS MEMO

This RFC discusses standardizing the protocol used in subnetted
environments in the ARPA-Internet. Distribution of this memo is
unlimited.

The author of this RFC is the Gateway Algorithms and Data Structures
(GADS) Task Force, chaired by David L. Mills.

INTRODUCTION

Several sites now contain a complex of local links connected to the
Internet via a gateway. The details of the internal connectivity are
of little interest to the rest of the Internet.

One way of organizing these local complexes of links is to use the
same strategy as the Internet uses to organize networks, that is, to
declare each link to be an entity (like a network) and to
interconnect the links with devices that perform routing functions
(like gateways). This general scheme is called subnetting, the
individual links are called subnets, and the connecting devices are
called subgateways (or bridges, or gateways).

All hosts in the Internet must make a decision when sending a
datagram, that is, they must answer the question "Is this datagram
addressed to a host on a directly connected network, or must it be
sent to a gateway?". In a subnetted environment, this question is
extended to "Is this datagram addressed to a host on a directly
connected subnet, or must it be sent to a (sub)gateway?". Let us
call answering this question "making the routing decision".

Because the hosts used in a subnetted environment must implement in
their IP or network interface software procedures for making the
routing decision, and because such hosts may be acquired from various
sources, it is important that a standard subnetting scheme be
identified so that different suppliers can provide compatible hosts
(that is, hosts compatible with the complexes at different sites and
each other). Without a designated standard for a subnetting scheme
suppliers can not create compatible hosts.

The potential problem is that if different subnetting schemes are
developed by different suppliers a customer that installs hosts from
two or more suppliers may find that they do not work together.



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RFC 940 April 1985
Toward an Internet Standard Scheme for Subnetting


This topic has been discussed in a set of RFCs [1,2,3,4] and in a
flurry of messages in the Gateway Algorithms and Data Structures Task
Force. It is strongly suggested that if subnetting is used at all,
it be according this new standard scheme.

APPROACH

An Internet address currently consists of a two-layer hierarchy, a
'network' and a per-network 'rest' field. This subnet scheme adds an
optional 'subnet' layer and field.

The subnet field is created by stealing some bits from the rest (or
host) field of the address. The details of the subnet field are site
specific. All three classes (A, B, and C) of networks may be
subnetted.

The use of subnets is an optional local decision. The fact that a
network has subnets is invisible outside that network, and the change
is local and can be instituted at a site without any global Internet
perturbations. A complex of links is assigned a single IP network
number, and outside that complex it appears as a single network with
that number. Only inside does local structure appear.

However, while the decision to use subnets at a site is optional, any
IP implementation which may possibly be used in a potentially
subnetted environment, should provide for subnet field configuration
as described above. Such an implementation will function properly in
environments with or without subnetting. On the other hand,
implementations lacking this provision will not function in a
subnetted environment, and are thus potentially less useful.

This specifications is not intended to require a particular
implementation technique inside the host, but rather to define the
external behavior of the host in a subnetted environment. It does
not specify how routing is done or the details of host construction.
Note that gateways are hosts, too.

However, it seems easiest to explain the approach by describing one
possible host implementation.

Example Implementation:

Let us use "subnet" to mean the locally attached transmission
medium.

The key decision to be made is "Is the destination IP address



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RFC 940 April 1985
Toward an Internet Standard Scheme for Subnetting


on my subnet or not?". Once this decision is made the host
knows to whether to send the datagram directly to the
destination on the subnet or to send the datagram to a gateway.

The host uses a 32-bit mask, along with the host's own IP
address, to determine whether or not destination IP addresses
are on its subnet.

The mask can be configured at boot time as a static quantity or
distributed by a protocol that is beyond the scope of this
memo.

If the bitwise AND of the mask with the destination IP address
matches the bitwise AND of the mask with the host's own IP
address, the destination is assumed on its subnet; if not, the
destination is assumed on a subnet or network reachable only
via a gateway.

Note: if the mask is all zeros, all destinations will appear
to be on this subnet; while, if the mask is all ones, only
the sending host itself will appear to be on this subnet.
If the mask contains ones in the network field and zeros in
the rest field, subnets are not in use.

The above procedure must be treated as a per interface
procedure for multihomed hosts.

For further information on background and rationale, see RFC-917,
"Internet Subnets" [1].

REFERENCES

[1] Mogul, J., "Internet Subnets", RFC-917, Stanford University,
October 1984.

[2] Postel, J., "Multi-LAN Address Resolution", RFC-925,
USC/Information Sciences Institute, October 1984.

[3] Clark, D., "A Subnetwork Addressing Scheme", RFC-932, MIT LCS,
January 1985.

[4] Karels, M., "Another Internet Subnet Addressing Scheme",
RFC-936, UC Berkeley, February 1985.






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