Internet Standard Subnetting Procedure
RFC 950
Document | Type |
RFC - Internet Standard
(August 1985; No errata)
Updated by RFC 6918
Updates RFC 792
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Authors | |||
Last updated | 2013-03-02 | ||
Stream | Legacy | ||
Formats | plain text html pdf htmlized bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 950 (Internet Standard) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
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.
Overview
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].
Acknowledgment
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 the
hosts on a LAN to monopolize most of the bandwidth. A common
solution to this problem is to divide the hosts into cliques of
high mutual communication, and put these cliques on separate
cables.
- Point-to-Point links: Sometimes a "local area", such as a
university campus, is split into two locations too far apart to
connect using the preferred LAN technology. In this case,
high-speed point-to-point links might connect several LANs.
An organization that has been forced to use more than one LAN has
three choices for assigning Internet addresses:
Mogul & Postel [Page 2]
RFC 950 August 1985
Internet Standard Subnetting Procedure
1. Acquire a distinct Internet network number for each cable;
subnets are not used at all.
2. Use a single network number for the entire organization, but
assign host numbers without regard to which LAN a host is on
("transparent subnets").
3. Use a single network number, and partition the host address
space by assigning subnet numbers to the LANs ("explicit
subnets").
Each of these approaches has disadvantages. The first, although not
requiring any new or modified protocols, results in an explosion in
the size of Internet routing tables. Information about the internal
details of local connectivity is propagated everywhere, although it
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