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Internet Standard Subnetting Procedure (RFC0950) Disclosure Number: IPCOM000004946D
Original Publication Date: 1985-Aug-01
Included in the Prior Art Database: 2001-Jul-12
Document File: 19 page(s) / 38K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

J.C. Mogul: AUTHOR [+2]


Status Of This Memo

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

Network Working Group J. Mogul (Stanford) Request for Comments: 950 J. Postel (ISI)

August 1985

Internet Standard Subnetting Procedure

Status Of This Memo

This RFC specifies a protocol for the ARPA-Internet community. If subnetting is implemented it is strongly recommended that these procedures be followed. Distribution of this memo is unlimited.


This memo discusses the utility of "subnets" of Internet networks, which are logically visible sub-sections of a single Internet network. For administrative or technical reasons, many organizations have chosen to divide one Internet network into several subnets, instead of acquiring a set of Internet network numbers. This memo specifies procedures for the use of subnets. These procedures are for hosts (e.g., workstations). The procedures used in and between subnet gateways are not fully described. Important motivation and background information for a subnetting standard is provided in RFC-940 [7].


This memo is based on RFC-917 [1]. Many people contributed to the development of the concepts described here. J. Noel Chiappa, Chris Kent, and Tim Mann, in particular, provided important suggestions. Additional contributions in shaping this memo were made by Zaw-Sing Su, Mike Karels, and the Gateway Algorithms and Data Structures Task Force (GADS).

Mogul Postel [Page 1]

RFC 950 August 1985 Internet Standard Subnetting Procedure

1. Motivation

The original view of the Internet universe was a two-level hierarchy: the top level the Internet as a whole, and the level below it individual networks, each with its own network number. The Internet does not have a hierarchical topology, rather the interpretation of addresses is hierarchical. In this two-level model, each host sees its network as a single entity; that is, the network may be treated as a "black box" to which a set of hosts is connected.

While this view has proved simple and powerful, a number of organizations have found it inadequate, and have added a third level to the interpretation of Internet addresses. In this view, a given Internet network is divided into a collection of subnets.

The three-level model is useful in networks belonging to moderately large organizations (e.g., Universities or companies with more than one building), where it is often necessary to use more than one LAN cable to cover a "local area". Each LAN may then be treated as a subnet.

There are several reasons why an organization might use more than one cable to cover a campus:

Different technologies: Especially in a research environment, there may be more than one kind of LAN in use; e.g., an organization may have some equipment that supports Ethernet, and some that supports a ring network.

Limits of technologies: Most LAN technologies impose limits, based on electrical parameters, on the number of hosts connected, and on the total length of the cable. It is easy to exceed these limits, especially those on cable length.

Network congestion: It is possible for a small subset of t...