Internet Standard Subnetting Procedure

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).

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:

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
   is of little or no use outside the local organization.  Especially as
   some current gateway implementations do not have much space for
   routing tables, it would be good to avoid this problem.

   The second approach requires some convention or protocol that makes
   the collection of LANs appear to be a single Internet network.  For
   example, this can be done on LANs where each Internet address is
   translated to a hardware address using an Address Resolution Protocol
   (ARP), by having the bridges between the LANs intercept ARP requests
   for non-local targets, see RFC-925 [2].  However, it is not possible
   to do this for all LAN technologies, especially those where ARP
   protocols are not currently used, or if the LAN does not support
   broadcasts.  A more fundamental problem is that bridges must discover
   which LAN a host is on, perhaps by using a broadcast algorithm.  As
   the number of LANs grows, the cost of broadcasting grows as well;
   also, the size of translation caches required in the bridges grows
   with the total number of hosts in the network.

   The third approach is to explicitly support subnets.  This does have
   a disadvantage, in that it is a modification of the Internet
   Protocol, and thus requires changes to IP implementations already in
   use (if these implementations are to be used on a subnetted network).
   However, these changes are relatively minor, and once made, yield a
   simple and efficient solution to the problem.  Also, the approach
   avoids any changes that would be incompatible with existing hosts on
   non-subnetted networks.

   Further, when appropriate design choices are made, it is possible for
   hosts which believe they are on a non-subnetted network to be used on
   a subnetted one, as explained in RFC-917 [1].  This is useful when it
   is not possible to modify some of the hosts to support subnets
   explicitly, or when a gradual transition is preferred.

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2.  Standards for Subnet Addressing

   This section first describes a proposal for interpretation of
   Internet addresses to support subnets.  Next it discusses changes to
   host software to support subnets.  Finally, it presents a procedures
   for discovering what address interpretation is in use on a given
   network (i.e., what address mask is in use).

   2.1. Interpretation of Internet Addresses

      Suppose that an organization has been assigned an Internet network
      number, has further divided that network into a set of subnets,
      and wants to assign host addresses: how should this be done?
      Since there are minimal restrictions on the assignment of the
      "local address" part of the Internet address, several approaches
      have been proposed for representing the subnet number:

         1. Variable-width field:  Any number of the bits of the local
            address part are used for the subnet number; the size of
            this field, although constant for a given network, varies
            from network to network.  If the field width is zero, then
            subnets are not in use.

         2. Fixed-width field:  A specific number of bits (e.g., eight)
            is used for the subnet number, if subnets are in use.

         3. Self-encoding variable-width field:  Just as the width
            (i.e., class) of the network number field is encoded by its
            high-order bits, the width of the subnet field is similarly
            encoded.

         4. Self-encoding fixed-width field:  A specific number of bits
            is used for the subnet number.

         5. Masked bits:  Use a bit mask ("address mask") to identify
            which bits of the local address field indicate the subnet
            number.

      What criteria can be used to choose one of these five schemes?
      First, should we use a self-encoding scheme?  And, should it be
      possible to tell from examining an Internet address if it refers
      to a subnetted network, without reference to any other
      information?

         An interesting feature of self-encoding is that it allows the

Internet Standard Subnetting Procedure


         address space of a network to be divided into subnets of
         different sizes, typically one subnet of half the address space
         and a set of small subnets.

            For example, consider a class C network that uses a
            self-encoding scheme with one bit to indicate if it is the
            large subnet or not and an additional three bits to identify
            the small subnet.  If the first bit is zero then this is the
            large subnet, if the first bit is one then the following
            bits (3 in this example) give the subnet number.  There is
            one subnet with 128 host addresses, and eight subnets with
            16 hosts each.

         To establish a subnetting standard the parameters and
         interpretation of the self-encoding scheme must be fixed and
         consistent throughout the Internet.

         It could be assumed that all networks are subnetted.  This
         would allow addresses to be interpreted without reference to
         any other information.

            This is a significant advantage, that given the Internet
            address no additional information is needed for an
            implementation to determine if two addresses are on the same
            subnet.  However, this can also be viewed as a disadvantage:
            it may cause problems for networks which have existing host
            numbers that use arbitrary bits in the local address part.
            In other words, it is useful to be able to control whether a
            network is subnetted independently from the assignment of
            host addresses.

         The alternative is to have the fact that a network is subnetted
         kept separate from the address.  If one finds, somehow, that
         the network is subnetted then the standard self-encoded
         subnetted network address rules are followed, otherwise the
         non-subnetted network addressing rules are followed.

      If a self-encoding scheme is not used, there is no reason to use a
      fixed-width field scheme: since there must in any case be some
      per-network "flag" to indicate if subnets are in use, the
      additional cost of using an integer (a subnet field width or
      address mask) instead of a boolean is negligible.  The advantage
      of using the address mask scheme is that it allows each
      organization to choose the best way to allocate relatively scarce
      bits of local address to subnet and host numbers.  Therefore, we
      choose the address-mask scheme: it is the most flexible scheme,
      yet costs no more to implement than any other.

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      For example, the Internet address might be interpreted as:

         <network-number><subnet-number><host-number>

      where the <network-number> field is as defined by IP [3], the
      <host-number> field is at least 1-bit wide, and the width of the
      <subnet-number> field is constant for a given network.  No further
      structure is required for the <subnet-number> or <host-number>
      fields.  If the width of the <subnet-number> field is zero, then
      the network is not subnetted (i.e., the interpretation of [3] is
      used).

      For example, on a Class B network with a 6-bit wide subnet field,
      an address would be broken down like this:

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1 0|        NETWORK            |  SUBNET   |    Host Number    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Since the bits that identify the subnet are specified by a
      bitmask, they need not be adjacent in the address.  However, we
      recommend that the subnet bits be contiguous and located as the
      most significant bits of the local address.

      Special Addresses:

         From the Assigned Numbers memo [9]:

            "In certain contexts, it is useful to have fixed addresses
            with functional significance rather than as identifiers of
            specific hosts.  When such usage is called for, the address
            zero is to be interpreted as meaning "this", as in "this
            network".  The address of all ones are to be interpreted as
            meaning "all", as in "all hosts".  For example, the address
            128.9.255.255 could be interpreted as meaning all hosts on
            the network 128.9.  Or, the address 0.0.0.37 could be
            interpreted as meaning host 37 on this network."

         It is useful to preserve and extend the interpretation of these
         special addresses in subnetted networks.  This means the values
         of all zeros and all ones in the subnet field should not be
         assigned to actual (physical) subnets.

            In the example above, the 6-bit wide subnet field may have
            any value except 0 and 63.

Internet Standard Subnetting Procedure


         Please note that there is no effect or new restriction on the
         addresses of hosts on non-subnetted networks.

   2.2. Changes to Host Software to Support Subnets

      In most implementations of IP, there is code in the module that
      handles outgoing datagrams to decide if a datagram can be sent
      directly to the destination on the local network or if it must be
      sent to a gateway.

      Generally the code is something like this:

         IF ip_net_number(dg.ip_dest) = ip_net_number(my_ip_addr)
             THEN
                 send_dg_locally(dg, dg.ip_dest)
             ELSE
                 send_dg_locally(dg,
                                  gateway_to(ip_net_number(dg.ip_dest)))

      (If the code supports multiply-connected networks, it will be more
      complicated, but this is irrelevant to the current discussion.)

      To support subnets, it is necessary to store one more 32-bit
      quantity, called my_ip_mask.  This is a bit-mask with bits set in
      the fields corresponding to the IP network number, and additional
      bits set corresponding to the subnet number field.

      The code then becomes:

         IF bitwise_and(dg.ip_dest, my_ip_mask)
                                   = bitwise_and(my_ip_addr, my_ip_mask)
             THEN
                 send_dg_locally(dg, dg.ip_dest)
             ELSE
                 send_dg_locally(dg,
                        gateway_to(bitwise_and(dg.ip_dest, my_ip_mask)))

      Of course, part of the expression in the conditional can be
      pre-computed.

      It may or may not be necessary to modify the "gateway_to"
      function, so that it too takes the subnet field bits into account
      when performing comparisons.

      To support multiply-connected hosts, the code can be changed to

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      keep  the "my_ip_addr" and "my_ip_mask" quantities on a
      per-interface basis; the expression in the conditional must then
      be evaluated for each interface.

   2.3. Finding the Address Mask

      How can a host determine what address mask is in use on a subnet
      to which it is connected?  The problem is analogous to several
      other "bootstrapping" problems for Internet hosts: how a host
      determines its own address, and how it locates a gateway on its
      local network.  In all three cases, there are two basic solutions:
      "hardwired" information, and broadcast-based protocols.

      Hardwired information is that available to a host in isolation
      from a network.  It may be compiled-in, or (preferably) stored in
      a disk file.  However, for the increasingly common case of a
      diskless workstation that is bootloaded over a LAN, neither
      hardwired solution is satisfactory.

      Instead, since most LAN technology supports broadcasting, a better
      method is for the newly-booted host to broadcast a request for the
      necessary information.  For example, for the purpose of
      determining its Internet address, a host may use the "Reverse
      Address Resolution Protocol" (RARP) [4].

      However, since a newly-booted host usually needs to gather several
      facts (e.g., its IP address, the hardware address of a gateway,
      the IP address of a domain name server, the subnet address mask),
      it would be better to acquire all this information in one request
      if possible, rather than doing numerous broadcasts on the network.
      The mechanisms designed to boot diskless workstations can also
      load per-host specific configuration files that contain the
      required information (e.g., see RFC-951 [8]).  It is possible, and
      desirable, to obtain all the facts necessary to operate a host
      from a boot server using only one broadcast message.

      In the case where it is necessary for a host to find the address
      mask as a separate operation the following mechanism is provided:

         To provide the address mask information the ICMP protocol [5]
         is extended by adding a new pair of ICMP message types,
         "Address Mask Request" and "Address Mask Reply", analogous to
         the "Information Request" and "Information Reply" ICMP
         messages.  These are described in detail in Appendix I.

         The intended use of these new ICMP messages is that a host,
         when booting, broadcast an "Address Mask Request" message.  A

Internet Standard Subnetting Procedure


         gateway (or a host acting in lieu of a gateway) that receives
         this message responds with an "Address Mask Reply".  If there
         is no indication in the request which host sent it (i.e., the
         IP Source Address is zero), the reply is broadcast as well.
         The requesting host will hear the response, and from it
         determine the address mask.

         Since there is only one possible value that can be sent in an
         "Address Mask Reply" on any given LAN, there is no need for the
         requesting host to match the responses it hears against the
         request it sent; similarly, there is no problem if more than
         one gateway responds.  We assume that hosts reboot
         infrequently, so the broadcast load on a network from use of
         this protocol should be small.

      If a host is connected to more than one LAN, it might have to find
      the address mask for each.

      One potential problem is what a host should do if it can not find
      out the address mask, even after a reasonable number of tries.
      Three interpretations can be placed on the situation:

         1. The local net exists in (permanent) isolation from all other
            nets.

         2. Subnets are not in use, and no host can supply the address
            mask.

         3. All gateways on the local net are (temporarily) down.

      The first and second situations imply that the address mask is
      identical with the Internet network number mask.  In the third
      situation, there is no way to determine what the proper value is;
      the safest choice is thus a mask identical with the Internet
      network number mask.  Although this might later turn out to be
      wrong, it will not prevent transmissions that would otherwise
      succeed.  It is possible for a host to recover from a wrong
      choice: when a gateway comes up, it should broadcast an "Address
      Mask Reply"; when a host receives such a message that disagrees
      with its guess, it should change its mask to conform to the
      received value.  No host or gateway should send an "Address Mask
      Reply" based on a "guessed" value.

      Finally, note that no host is required to use this ICMP protocol
      to discover the address mask; it is perfectly reasonable for a
      host with non-volatile storage to use stored information
      (including a configuration file from a boot server).

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Appendix I.  Address Mask ICMP

   Address Mask Request or Address Mask Reply

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |      Code     |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Identifier          |       Sequence Number         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Address Mask                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      IP Fields:

         Addresses

            The address of the source in an address mask request message
            will be the destination of the address mask reply message.
            To form an address mask reply message, the source address of
            the request becomes the destination address of the reply,
            the source address of the reply is set to the replier's
            address, the type code changed to AM2, the address mask
            value inserted into the Address Mask field, and the checksum
            recomputed.  However, if the source address in the request
            message is zero, then the destination address for the reply
            message should denote a broadcast.

      ICMP Fields:

         Type

            AM1 for address mask request message

            AM2 for address mask reply message

         Code

            0 for address mask request message

            0 for address mask reply message

         Checksum

            The checksum is the 16-bit one's complement of the one's

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            complement sum of the ICMP message starting with the ICMP
            Type.  For computing the checksum, the checksum field should
            be zero.  This checksum may be replaced in the future.

         Identifier

            An identifier to aid in matching requests and replies, may
            be zero.

         Sequence Number

            A sequence number to aid in matching requests and replies,
            may be zero.

         Address Mask

            A 32-bit mask.

      Description

         A gateway receiving an address mask request should return it
         with the address mask field set to the 32-bit mask of the bits
         identifying the subnet and network, for the subnet on which the
         request was received.

         If the requesting host does not know its own IP address, it may
         leave the source field zero; the reply should then be
         broadcast.  However, this approach should be avoided if at all
         possible, since it increases the superfluous broadcast load on
         the network.  Even when the replies are broadcast, since there
         is only one possible address mask for a subnet, there is no
         need to match requests with replies.  The "Identifier" and
         "Sequence Number" fields can be ignored.

            Type AM1 may be received from a gateway or a host.

            Type AM2 may be received from a gateway, or a host acting in
            lieu of a gateway.

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Appendix II.  Examples

   These examples show how a host can find out the address mask using
   the ICMP Address Mask Request and Address Mask Reply messages.  For
   the following examples, assume that address 255.255.255.255 denotes
   "broadcast to this physical medium" [6].

   1.  A Class A Network Case

      For this case, assume that the requesting host is on class A
      network 36.0.0.0, has address 36.40.0.123, that there is a gateway
      at 36.40.0.62, and that a 8-bit wide subnet field is in use, that
      is, the address mask is 255.255.0.0.

      The most efficient method, and the one we recommend, is for a host
      to first discover its own address (perhaps using "RARP" [4]), and
      then to send the ICMP request to 255.255.255.255:

         Source address:          36.40.0.123
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      The gateway can then respond directly to the requesting host.

         Source address:          36.40.0.62
         Destination address:     36.40.0.123
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.0.0

      Suppose that 36.40.0.123 is a diskless workstation, and does not
      know even its own host number.  It could send the following
      datagram:

         Source address:          0.0.0.0
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      36.40.0.62 will hear the datagram, and should respond with this
      datagram:

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         Source address:          36.40.0.62
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.0.0

      Note that the gateway uses the narrowest possible broadcast to
      reply.  Even so, the over use of broadcasts presents an
      unnecessary load to all hosts on the subnet, and so the use of the
      "anonymous" (0.0.0.0) source address must be kept to a minimum.

      If broadcasting is not allowed, we assume that hosts have wired-in
      information about neighbor gateways; thus, 36.40.0.123 might send
      this datagram:

         Source address:          36.40.0.123
         Destination address:     36.40.0.62
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      36.40.0.62 should respond exactly as in the previous case.

         Source address:          36.40.0.62
         Destination address:     36.40.0.123
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.0.0

   2.  A Class B Network Case

      For this case, assume that the requesting host is on class B
      network 128.99.0.0, has address 128.99.4.123, that there is a
      gateway at 128.99.4.62, and that a 6-bit wide subnet field is in
      use, that is, the address mask is 255.255.252.0.

      The host sends the ICMP request to 255.255.255.255:

         Source address:          128.99.4.123
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

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      The gateway can then respond directly to the requesting host.

         Source address:          128.99.4.62
         Destination address:     128.99.4.123
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.252.0

      In the diskless workstation case the host sends:

         Source address:          0.0.0.0
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      128.99.4.62 will hear the datagram, and should respond with this
      datagram:

         Source address:          128.99.4.62
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.252.0

      If broadcasting is not allowed 128.99.4.123 sends:

         Source address:          128.99.4.123
         Destination address:     128.99.4.62
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      128.99.4.62 should respond exactly as in the previous case.

         Source address:          128.99.4.62
         Destination address:     128.99.4.123
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.252.0

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   3.  A Class C Network Case (illustrating non-contiguous subnet bits)

      For this case, assume that the requesting host is on class C
      network 192.1.127.0, has address 192.1.127.19, that there is a
      gateway at 192.1.127.50, and that on network an 3-bit subnet field
      is in use (01011000), that is, the address mask is 255.255.255.88.

      The host sends the ICMP request to 255.255.255.255:

         Source address:          192.1.127.19
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      The gateway can then respond directly to the requesting host.

         Source address:          192.1.127.50
         Destination address:     192.1.127.19
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.255.88.

      In the diskless workstation case the host sends:

         Source address:          0.0.0.0
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      192.1.127.50 will hear the datagram, and should respond with this
      datagram:

         Source address:          192.1.127.50
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.255.88.

      If broadcasting is not allowed 192.1.127.19 sends:

Internet Standard Subnetting Procedure


         Source address:          192.1.127.19
         Destination address:     192.1.127.50
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      192.1.127.50 should respond exactly as in the previous case.

         Source address:          192.1.127.50
         Destination address:     192.1.127.19
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.255.88

Appendix III.  Glossary

   Bridge

      A node connected to two or more administratively indistinguishable
      but physically distinct subnets, that automatically forwards
      datagrams when necessary, but whose existence is not known to
      other hosts.  Also called a "software repeater".

   Gateway

      A node connected to two or more administratively distinct networks
      and/or subnets, to which hosts send datagrams to be forwarded.

   Host Field

      The bit field in an Internet address used for denoting a specific
      host.

   Internet

      The collection of connected networks using the IP protocol.

   Local Address

      The rest field of the Internet address (as defined in [3]).

   Network

      A single Internet network (which may or may not be divided into
      subnets).

Internet Standard Subnetting Procedure


   Network Number

      The network field of the Internet address.

   Subnet

      One or more physical networks forming a subset of an Internet
      network.  A subnet is explicitly identified in the Internet
      address.

   Subnet Field

      The bit field in an Internet address denoting the subnet number.
      The bits making up this field are not necessarily contiguous in
      the address.

   Subnet Number

      A number identifying a subnet within a network.

Appendix IV.  Assigned Numbers

   The following assignments are made for protocol parameters used in
   the support of subnets.  The only assignments needed are for the
   Internet Control Message Protocol (ICMP) [5].

   ICMP Message Types

      AM1 = 17

      AM2 = 18

Internet Standard Subnetting Procedure


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]  Postel, J., "Internet Protocol", RFC-791, USC/Information
        Sciences Institute, September 1981.

   [4]  Finlayson, R., T. Mann, J. Mogul, M. Theimer, "A Reverse Address
        Resolution Protocol", RFC-903, Stanford University, June 1984.

   [5]  Postel, J., "Internet Control Message Protocol", RFC-792,
        USC/Information Sciences Institute, September 1981.

   [6]  Mogul, J., "Broadcasting Internet Datagrams", RFC-919, Stanford
        University, October 1984.

   [7]  GADS, "Towards an Internet Standard Scheme for Subnetting",
        RFC-940, Network Information Center, SRI International,
        April 1985.

   [8]  Croft, B., and J. Gilmore, "BOOTP -- UDP Bootstrap Protocol",
        RFC-951, Stanford University, August 1985.

   [9]  Reynolds, J., and J. Postel, "Assigned Numbers", RFC-943,
        USC/Information Sciences Institute, April 1985.