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On the assignment of subnet numbers (RFC1219) Disclosure Number: IPCOM000002033D
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2000-Sep-12
Document File: 11 page(s) / 28K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

P.F. Tsuchiya: AUTHOR


Status Of This Memo

This text was extracted from a ASCII document.
This is the abbreviated version, containing approximately 10% of the total text.

Network Working Group P. Tsuchiya

Request for Comments: 1219 Bellcore

April 1991

On the Assignment of Subnet Numbers

Status Of This Memo

This memo suggests a new procedure for assigning subnet numbers. Use

of this assignment technique within a network would be a purely local

matter, and would not effect other networks. Therefore, the use of

these procedures is entirely discretionary.

This memo provides information for the Internet community. It does

not specify an Internet standard. Distribution of this memo is



RFC-950 [2] specifies a procedure for subnetting Internet addresses

using a bit-mask. While RFC-950 allows the "ones" in the subnet mask

to be non-contiguous, RFC-950 recommends that 1) they be contiguous,

and 2) that they occupy the most significant bits of the "host" part

of the internet address.

RFC-950 did not specify whether different subnets of the same network

may have different masks. This ambiguity was unfortunate, as it

resulted in development of routing protocols that do not support

different masks; see e.g., RIP [6]. The Gateway Requirements RFC [7]

settled the issue in favor of allowing different masks, and therefore

future routing protocols may be expected to support this feature;

OSPF [3] is an example.

The network administrator must of course determine the mask for each

subnet. This involves making an estimate of how many hosts each

subnet is expected to have. As it is often impossible to predict how

large each subnet will grow, inefficient choices are often made, with

some subnets under-utilized, and others possibly requiring

renumbering because of exceeded capacity.

This memo specifies a procedure for assigning subnet numbers that

eliminates the need to estimate subnet size. Essentially, host bits

(mask = 0) are assigned from the least significant bit working

towards the most, and subnet bits (mask = 1) are assigned from the

most significant bit working towards the least. As subnets grow,

more host bits are assigned. As the number of subnets grows, more

subnet bits are assigned. While this process does sometimes result

in new subnet masks, no host ever need change addresses.

This technique is not new, but it is also not widely known, and even

less widely implemented. With the development of new routing

protocols such as OSPF, it is possible to take full advantage of this

technique. The purpose of this memo, then, is to make this technique<...