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Network Working Group                                  Michael J. KarelsRequest for Comments: 936                                    UC Berkeley                                                           February 1985Another Internet Subnet Addressing SchemeStatus 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 inRFC-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 inRFC-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-localKarels                                                          [Page 1]

RFC 936                                                    February 1985Another 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 ofKarels                                                          [Page 2]

RFC 936                                                    February 1985Another 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 ofRFC-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 comeKarels                                                          [Page 3]

RFC 936                                                    February 1985Another Internet Subnet Addressing Scheme   up.  Also, the recovery procedure proposed byRFC-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 1985Karels                                                          [Page 4]