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rfc:rfc936

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

Karels [Page 2]

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

Karels [Page 4]

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