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RFC 936
Network Working Group Michael J. Karels
Request for Comments: 936 UC Berkeley
February 1985
Another Internet Subnet 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.
Introduction
There have been several proposals for schemes to allow the use of a
single Internet network number to refer to a collection of physical
networks under common administration which are reachable from the
rest of the Internet by a common route. Such schemes allow a
simplified view of an otherwise complicated topology from hosts and
gateways outside of this collection. They allow the complexity of
the number and type of these networks, and routing to them, to be
localized. Additions and changes in configuration thus cause no
detectable change, and no interruption of service, due to slow
propagation of routing and other information outside of the local
environment. These schemes also simplify the administration of the
network, as changes do not require allocation of new network numbers
for each new cable installed. The motivation for explicit or
implicit subnets, several of the alternatives, and descriptions of
existing implementations of this type have been described in detail
[1,2]. This proposal discusses an alternative scheme, one that has
been in use at the University of California, Berkeley since
April 1984.
Subnet Addressing at Berkeley
As in the proposal by Jeff Mogul in RFC-917, the Berkeley subnet
addressing utilizes encoding of the host part of the Internet
address. Hosts and gateways on the local network are able to
determine the subnet number from each local address, and then route
local packets based on the subnet number. Logically, the collection
of subnets appears to external sites to be a single, homogenous
network. Internally, however, each subnet is distinguished from the
others and from other networks, and internal routing decisions are
based on the subnet rather than the network number.
The encoding of subnet addresses is similar to that proposed in
RFC-917. In decomposing an Internet address into the network and
host parts, the algorithm is modified if the network is "local", that
is, if the network is a directly-connected network under local
administrative control. (Networks are marked as local or non-local
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RFC 936 February 1985
Another Internet Subnet Addressing Scheme
at the time each network interface's address is set at boot time.)
For local addresses, the host part is examined for a subnet number.
Local addresses may be on the main network, or they may be on a
subnet. The high-order bit of the host number is used to distinguish
between subnets and the main net. If the high-order bit of the host
field is set, then the remainder of the high-order byte of the host
part is taken to be the subnet number. If the high-order bit is
clear, then the address is interpreted in the normal fashion. For
Class A networks, using 8-bit subnet fields, this allows a network
with up to 127 subnets, each of 65535 hosts maximum, and a main net
with 2^23 hosts. Class B nets may include 127 subnets, each of up to
255 hosts, and 32767 hosts on the main net. Class C networks are not
currently included in this scheme. They might be reasonably be added,
using four bits of the host part for a subnet desgination and four
bits for the host, allowing 8 subnets of 15 hosts and 126 hosts on
the main net.
The current implementation does not use subnet numbers separately
from the network field, but instead treats the subnet field as an
extension of the network field. Functions that previously returned
the network number from an address now return a network or
network-subnetwork number. Conveniently, Class A subnets are
distinguishable from Class B networks, although each is a 16-bit
quantity, and Class B subnets are disjoint with Class C network
numbers. The net result is that subnets appear to be separate,
independent networks with their own routing entries within the
network, but outside of the network, they are invisible. There is no
current facility at Berkeley for broadcasting on the logical network;
broadcasting may be done on each subnet that uses harware capable of
broadcast.
Discussion
There have been several earlier proposals for methods of allowing
several physical networks to share an Internet network designation,
and to provide routing within this logical network. RFC-917 proposes
a means for encoding the host part of each local address such that
the hosts, or the gateways connecting them, are able to determine the
physical network for the host. The current proposal is most similar
to that scheme; the differences are discussed in detail below.
Another proposal (RFC-925) involves the use of intelligent gateways
to perform routing for unmodified hosts, using an Address Resolution
Protocol (ARP) [2]. This has the advantage of placing all
modifications in the gateways, but is likely to require additional
routing protocols and caching mechanisms in the gateways in order to
avoid excessive broadcasts for address resolution. A modification of
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RFC 936 February 1985
Another Internet Subnet Addressing Scheme
this method is to perform encoding of subnets within host addresses
by convention to simplify the routing in the gateways, without
modifying host software to recognize these subnet addresses. These
techniques were not considered for use at Berkeley, because all
packet forwarding was being done by multi- homed hosts, all of which
ran the same software as the singly-homed hosts (4.2BSD Unix).
The most recent proposal, RFC-932 [3], provides subnetting by
encoding the network part of the Internet address rather than the
host part. Ordinary hosts need not know of this convention,
eliminating the need for modification to host software. Gateways
would be able to take advantage of this encoding to compress the
routing information for the collection of networks into a single
entry. Unfortunately, implementation of that scheme would require a
fairly concerted transition by the gateways of the Internet, or the
transition period would be likely to overflow the routing tables in
the existing gateways. All of the hosts on the larger networks would
be forced to change addresses from their current Class A or B
addresses to "B 1/2" addresses. There are a limited number (4096) of
blocks of Class C addresses available using this encoding. The
number of universities and other organizations having already
implemented subnets or contemplating their installation argues for a
more extensible scheme, as well as one that can be implemented more
quickly.
The current proposal is most similar to that of RFC-917; indeed, the
two implementations are nearly compatible. There are two differences
of significance. First, the use of a bit to distinguish subnetted
addresses from non-subnetted addresses allows both smaller subnets
and a larger (physical or logical) main network. Half of the host
addresses within a Class A or B network are reserved for use in
subnets, the other half are available for the primary net. This may
useful when using a hardware medium that is capable of supporting
large numbers of hosts or for transparent subnetting (e.g. using
ARP-based bridges). The corresponding disadvantage is that fewer
subnets may be supported. The allocation of bits between the subnet
number and the host field could be adjusted, but for Class B
networks, neither is excessively large. Given the limited address
space of the current Internet addressing, this is a difficult choice.
The second difference is that the width of the subnet field is fixed
in advance. This simplifies the already-too-complicated code to
interpret Internet addresses, and avoids the bootstrap problem. If
the subnet field width is to be determined dynamically, some fraction
of the hosts on a network must be prepared to specify this value, and
the situation will be unworkable if one of these hosts does not make
the correct choice or none are accessible when other machines come
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RFC 936 February 1985
Another Internet Subnet Addressing Scheme
up. Also, the recovery procedure proposed by RFC-917 seems
unnecessarily complicated and liable to fail. Dynamic discovery of
this value depends on another modification as well, the addition of a
new ICMP request. The alternatives are to specify the field size as
a standard, or to require each implementation to be configurable in
advance (e.g with a system compilation option or the use of a system
patch installed when a host is initially installed. The use of a
standard field width seems preferable, and an 8-bit field allows the
most efficient implementations on most architectures. For Class C
nets, a 4-bit field seems the only choice for a standard division.
References
[1] J. Mogul, "Internet Subnets", RFC-917, Stanford University,
October 1984
[2] J. Postel, "Multi-LAN Address Resolution", RFC-925, USC-ISI,
October 1984
[3] D. Clark, "A Subnet Addressing Scheme", RFC-932, MIT-LCS,
January 1985
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