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Network Working Group GADS Request for Comments: 940

                                                            April 1985

         Toward an Internet Standard Scheme for Subnetting

STATUS OF THIS MEMO

 This RFC discusses standardizing the protocol used in subnetted
 environments in the ARPA-Internet.  Distribution of this memo is
 unlimited.

 The author of this RFC is the Gateway Algorithms and Data Structures
 (GADS) Task Force, chaired by David L. Mills.

INTRODUCTION

 Several sites now contain a complex of local links connected to the
 Internet via a gateway.  The details of the internal connectivity are
 of little interest to the rest of the Internet.

 One way of organizing these local complexes of links is to use the
 same strategy as the Internet uses to organize networks, that is, to
 declare each link to be an entity (like a network) and to
 interconnect the links with devices that perform routing functions
 (like gateways).  This general scheme is called subnetting, the
 individual links are called subnets, and the connecting devices are
 called subgateways (or bridges, or gateways).

 All hosts in the Internet must make a decision when sending a
 datagram, that is, they must answer the question "Is this datagram
 addressed to a host on a directly connected network, or must it be
 sent to a gateway?".  In a subnetted environment, this question is
 extended to "Is this datagram addressed to a host on a directly
 connected subnet, or must it be sent to a (sub)gateway?".  Let us
 call answering this question "making the routing decision".

 Because the hosts used in a subnetted environment must implement in
 their IP or network interface software procedures for making the
 routing decision, and because such hosts may be acquired from various
 sources, it is important that a standard subnetting scheme be
 identified so that different suppliers can provide compatible hosts
 (that is, hosts compatible with the complexes at different sites and
 each other).  Without a designated standard for a subnetting scheme
 suppliers can not create compatible hosts.

 The potential problem is that if different subnetting schemes are
 developed by different suppliers a customer that installs hosts from
 two or more suppliers may find that they do not work together.

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RFC 940 April 1985 Toward an Internet Standard Scheme for Subnetting

 This topic has been discussed in a set of RFCs [1,2,3,4] and in a
 flurry of messages in the Gateway Algorithms and Data Structures Task
 Force.  It is strongly suggested that if subnetting is used at all,
 it be according this new standard scheme.

APPROACH

 An Internet address currently consists of a two-layer hierarchy, a
 'network' and a per-network 'rest' field.  This subnet scheme adds an
 optional 'subnet' layer and field.

 The subnet field is created by stealing some bits from the rest (or
 host) field of the address.  The details of the subnet field are site
 specific.  All three classes (A, B, and C) of networks may be
 subnetted.

 The use of subnets is an optional local decision.  The fact that a
 network has subnets is invisible outside that network, and the change
 is local and can be instituted at a site without any global Internet
 perturbations.  A complex of links is assigned a single IP network
 number, and outside that complex it appears as a single network with
 that number.  Only inside does local structure appear.

 However, while the decision to use subnets at a site is optional, any
 IP implementation which may possibly be used in a potentially
 subnetted environment, should provide for subnet field configuration
 as described above.  Such an implementation will function properly in
 environments with or without subnetting.  On the other hand,
 implementations lacking this provision will not function in a
 subnetted environment, and are thus potentially less useful.

 This specifications is not intended to require a particular
 implementation technique inside the host, but rather to define the
 external behavior of the host in a subnetted environment.  It does
 not specify how routing is done or the details of host construction.
 Note that gateways are hosts, too.

 However, it seems easiest to explain the approach by describing one
 possible host implementation.

    Example Implementation:

       Let us use "subnet" to mean the locally attached transmission
       medium.

       The key decision to be made is "Is the destination IP address

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RFC 940 April 1985 Toward an Internet Standard Scheme for Subnetting

       on my subnet or not?".  Once this decision is made the host
       knows to whether to send the datagram directly to the
       destination on the subnet or to send the datagram to a gateway.

       The host uses a 32-bit mask, along with the host's own IP
       address, to determine whether or not destination IP addresses
       are on its subnet.

       The mask can be configured at boot time as a static quantity or
       distributed by a protocol that is beyond the scope of this
       memo.

       If the bitwise AND of the mask with the destination IP address
       matches the bitwise AND of the mask with the host's own IP
       address, the destination is assumed on its subnet; if not, the
       destination is assumed on a subnet or network reachable only
       via a gateway.

          Note: if the mask is all zeros, all destinations will appear
          to be on this subnet; while, if the mask is all ones, only
          the sending host itself will appear to be on this subnet.
          If the mask contains ones in the network field and zeros in
          the rest field, subnets are not in use.

       The above procedure must be treated as a per interface
       procedure for multihomed hosts.

 For further information on background and rationale, see RFC-917,
 "Internet Subnets" [1].

REFERENCES

 [1]  Mogul, J., "Internet Subnets", RFC-917, Stanford University,
 October 1984.

 [2]  Postel, J., "Multi-LAN Address Resolution", RFC-925,
 USC/Information Sciences Institute, October 1984.

 [3]  Clark, D., "A Subnetwork Addressing Scheme", RFC-932, MIT LCS,
 January 1985.

 [4]  Karels, M., "Another Internet Subnet Addressing Scheme",
 RFC-936, UC Berkeley, February 1985.

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