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rfc:rfc917

Network Working Group Jeffrey Mogul Request for Comments: 917 Computer Science Department

                                                   Stanford University
                                                          October 1984
                          INTERNET SUBNETS

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.

Overview

 We discuss 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.
 We propose procedures for the use of subnets, and discuss approaches
 to solving the problems that arise, particularly that of routing.

Acknowledgment

 This proposal is the result of discussion with several other people.
 J. Noel Chiappa, Chris Kent, and Tim Mann, in particular, provided
 important suggestions.

1. Introduction

 The original view of the Internet universe was a two-level hierarchy:
 the top level the catenet as a whole, and the level below it a
 collection of "Internet Networks", each with its own Network Number.
 (We do not mean that the Internet has a hierarchical topology, but
 that the interpretation of addresses is hierarchical.)
 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 might (or might not) be divided into a collection of
 subnets.
 The original, two-level, view carries a strong presumption that, to a
 host on an Internet network, that network may be viewed as a single
 edge; to put it another way, the network may be treated as a "black
 box" to which a set of hosts is connected.  This is true of the

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 ARPANET, because the IMPs mask the use of specific links in that
 network.  It is also true of most local area network (LAN)
 technologies, such as Ethernet or ring networks.
 However, this presumption fails in many practical cases, because in
 moderately large organizations (e.g., Universities or companies with
 more than one building) it is often necessary to use more than one
 LAN cable to cover a "local area".  For example, at this writing
 there are eighteen such cables in use at Stanford University, with
 more planned.
 There are several reasons why an organization might use more than one
 cable to cover a campus:
  1. 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.
  1. Limits of technologies: Most LAN technologies impose limits,

based 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.
  1. 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.
  1. 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:
    1. Acquire a distinct Internet network number for each cable.
    2. Use a single network number for the entire organization, but
       assign host numbers without regard to which LAN a host is on.
       (We will call this choice "transparent subnets".)
    3. Use a single network number, and partition the host address
       space by assigning subnet numbers to the LANs. ("Explicit
       subnets".)

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 Each of these approaches has disadvantages.  The first, although not
 requiring any new or modified protocols, does result 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 nice 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.  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 addresses the key problem: existing standards
 assume that all hosts on an Internet local network are on a single
 cable.  The solution 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, we believe that these changes are relatively minor, and once
 made, yield a simple and efficient solution to the problem.  Also,
 the approach we take in this document is to avoid 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 will be explained later.  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.  Because of
 this, there seems little reason to use the second approach listed
 above.
 The rest of this document describes approaches to subnets of Internet
 Networks.

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 1.1. Terminology
    To avoid either ambiguity or prolixity, we will define a few
    terms, which will be used in the following sections:
    Catenet
       The collection of connected Internet Networks
    Network
       A single Internet network (that may or may not be divided into
       subnets.)
    Subnet
       A subnet of an Internet network.
    Network Number
       As in [8].
    Local Address
       The bits in an Internet address not used for the network
       number; also known as "rest field".
    Subnet Number
       A number identifying a subnet within a network.
    Subnet Field
       The bit field in an Internet address used for the subnet
       number.
    Host Field
       The bit field in an Internet address used for denoting a
       specific host.
    Gateway
       A node connected to two or more administratively distinct
       networks and/or subnets, to which hosts send datagrams to be
       forwarded.

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    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 know to other hosts.  Also called a "software
       repeater".

2. Standards for Subnet Addressing

 Following the division presented in [2], we observe that subnets are
 fundamentally an issue of addressing.  In this section, we first
 describe a proposal for interpretation of Internet Addressing to
 support subnets.  We then discuss the interaction between this
 address format and broadcasting; finally, we present a protocol for
 discovering what address interpretation is in use on a given network.
 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 is used for the subnet number.  Subnets are in use if the
          high-order bit of this field is one; otherwise, the entire
          local address part is used for host number.
    Since there seems to be no advantage in doing otherwise, all these
    schemes place the subnet field as the most significant field in

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    the local address part.  Also, since the local address part of a
    Class C address is so small, there is little reason to support
    subnets of other than Class A and Class B networks.
    What criteria can we use to choose one of these four schemes?
    First, do we want to use a self-encoding scheme; that is, should
    it be possible to tell from examining an Internet address if it
    refers to a subnetted network, without reference to any other
    information?
    One advantage to self-encoding is that it allows one to determine
    if a non-local network has been divided into subnets.  It is not
    clear that this would be of any use.  The principle advantage,
    however, is that 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 non-subnetted networks which have existing
    host numbers that use arbitrary bits in the local address part
    <1>.  In other words, it is useful to be able control whether a
    network is subnetted independently from the assignment of host
    addresses.  Another disadvantage of any self-encoding scheme is
    that it reduces the local address space by at least a factor of
    two.
    If a self-encoding scheme is not used, it is clear that a
    variable-width subnet field is appropriate.  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 (the subnet field
    width) instead of a boolean is negligible.  The advantage of using
    a variable-width subnet field is that it allows each organization
    to choose the best way to allocate relatively scarce bits of local
    address to subnet and host numbers.
    Our proposal, therefore, is that the Internet address be
    interpreted as:
       <network-number><subnet-number><host-number>
    where the <network-number> field is as in [8], the <host-number>
    field is at least one 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 [8] is
    used.)

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    For example, on a Class A network with an eight bit wide subnet
    field, an address is 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0|    NETWORK    |     SUBNET    |         Host number         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    We expect that, for reasons of simplicity and efficient
    implementation, that most organizations will choose a subnet field
    width that is a multiple of eight bits.  However, an
    implementation must be prepared to handle other possible widths.
    We reject the use of "recursive subnets", the division of the host
    field into "sub-subnet" and host parts, because:
  1. There is no obvious need for a four-level hierarchy.
  1. The number of bits available in an IP address is not large

enough to make this useful in general.

  1. The extra mechanism required is complex.
 2.2. Changes to Host Software to Support Subnets
    In most implementations of IP, there is  code in the module that
    handles outgoing packet that does something like:
       IF ip_net_number(packet.ip_dest) = ip_net_number(my_ip_addr)
           THEN
               send_packet_locally(packet, packet.ip_dest)
           ELSE
               send_packet_locally(packet,
                  gateway_to(ip_net_number(packet.ip_dest)))
    (If the code supports multiple 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.  For example,
    on a Class A network using an eight-bit wide subnet field, the
    mask would be 255.255.0.0.
    The code then becomes:

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       IF bitwise_and(packet.ip_dest, my_ip_mask)
                        = bitwise_and(my_ip_addr, my_ip_mask)
           THEN
               send_packet_locally(packet, packet.ip_dest)
           ELSE
               send_packet_locally(packet,
                  gateway_to(bitwise_and(packet.ip_dest, my_ip_mask)))
    Of course, part of the expression in the conditionally can be
    pre-computed.
    It may or may not be necessary to modify the "gateway_to"
    function, so that it performs comparisons in the same way.
    To support multiply-connected hosts, the code can be changed to
    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. Subnets and Broadcasting
    In the absence of subnets, there are only two kinds of broadcast
    possible within the Internet Protocol <2>: broadcast to all hosts
    on a specific network, or broadcast to all hosts on "this
    network"; the latter is useful when a host does not know what
    network it is on.
    When subnets are used, the situation becomes slightly more
    complicated.  First, the possibility now exists of broadcasting to
    a specific subnet.  Second, broadcasting to all the hosts on a
    subnetted network requires additional mechanism; in [6] the use of
    "Reverse Path Forwarding" [3] is proposed.  Finally, the
    interpretation of a broadcast to "this network" is that it should
    not be forwarded outside of the original subnet.
    Implementations must therefore recognize three kinds of broadcast
    addresses, in addition to their own host addresses:
    This physical network
       A destination address of all ones (255.255.255.255) causes the
       a datagram to be sent as a broadcast on the local physical
       network; it must not be forwarded by any gateway.

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    Specific network
       The destination address contains a valid network number; the
       local address part is all ones (e.g., 36.255.255.255).
    Specific subnet
       The destination address contains a valid network number and a
       valid subnet number; the host field is all ones (e.g.,
       36.40.255.255).
    For further discussion of Internet broadcasting, see [6].
    One factor that may aid in deciding whether to use subnets is that
    it is possible to broadcast to all hosts of a subnetted network
    with a single operation at the originating host.  It is not
    possible to broadcast, in one step, to the same set of hosts if
    they are on distinct networks.
 2.4. Determining the Width of the Subnet Field
    How can a host (or gateway) determine what subnet field width is
    in use on a network 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
    hard-wired 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" [4].
    We propose to extend the ICMP protocol [9] by adding a new pair of
    ICMP message types, "Address Format Request" and "Address Format
    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 ICMPs is that a host, when booting,

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    broadcast an "Address Format Request" message <3>.  A gateway (or
    a host acting in lieu of a gateway) that receives this message
    responds with an "Address Format 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 width of
    the subnet field.
    Since there is only one possible value that can be sent in an
    "Address Format 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 must use this
    protocol on each, unless it can determine (from a response on one
    of the LANs) that several of the LANs are part of the same
    network, and thus must have the same subnet field width.
    One potential problem is what a host should do if it receives no
    response to its "Address Format Request", 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 supports this ICMP
          request.
       3. All gateways on the local net are (temporarily) down.
    The first and second situations imply that the subnet field width
    is zero.  In the third situation, there is no way to determine
    what the proper value is; the safest choice is thus zero.
    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 Format Reply"; when a
    host receives such a message that disagrees with its guess, it
    should adjust its data structures to conform to the received
    value.  No host or gateway should send an "Address Format Reply"
    based on a "guessed" value.

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    Finally, note that no host is required to use this ICMP protocol
    to discover the subnet field width; it is perfectly reasonable for
    a host with non-volatile storage to use stored information.

3. Subnet Routing Methods

 One problem that faces all Internet hosts is how to determine a route
 to another host.  In the presence of subnets, this problem is only
 slightly modified.
 The use of subnets means that there are two levels to the routing
 process, instead of one.  If the destination host is on the same
 network as the source host, the routing decision involves only the
 subnet gateways between the hosts.  If the destination is on a
 different network, then the routing decision requires the choice both
 of a gateway out of the source host's network, and of a route within
 the network to that gateway.
 Fortunately, many hosts can ignore this distinction (and, in fact,
 ignore all routing choices) by using a "default" gateway as the
 initial route to all destinations, and relying on ICMP Host Redirect
 messages to define more appropriate routes.  However, this is not an
 efficient method for a gateway or for a multi-homed host, since a
 redirect may not make up for a poor initial choice of route.  Such
 hosts should use a routing information exchange protocol, but that is
 beyond the scope of this document; in any case, the problem arises
 even when subnets are not used.
 The problem for a singly-connected host is thus to find at least one
 neighbor gateway.  Again, there are basic two solutions to this: use
 hard-wired information, or use broadcasts.  We believe that the
 neighbor-gateway acquisition problem is the same with or without
 subnets, and thus the choice of solution is not affected by the use
 of subnets.
 However, one problem remains: a source host must determine if
 datagram to a given destination address must be sent via a gateway,
 or sent directly to the destination host.  In other words, is the
 destination host on the same physical network as the source?  This
 particular phase of the routing process is the only one that requires
 an implementation to be explicitly aware of subnets; in fact, if
 broadcasts are not used, it is the only place where an Internet
 implementation must be modified to support subnets.
 Because of this, it is possible to use some existing implementations
 without modification in the presence of subnets <4>.  For this to
 work, such implementations must:

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  1. Be used only on singly-homed hosts, and not as a gateway.
  1. Be used on a broadcast LAN.
  1. Use an Address Resolution Protocol (ARP), such [7].
  1. Not be required to maintain connections in the case of gateway

crashes.

 In this case, one can modify the ARP server module in a subnet
 gateway so that when it receives an ARP request, it checks the target
 Internet address to see if it is along the best route to the target.
 If it is, it sends to the requesting host an ARP response indicating
 its own hardware address.  The requesting host thus believes that it
 knows the hardware address of the destination host, and sends packets
 to that address.  In fact, the packets are received by the gateway,
 and forwarded to the destination host by the usual means.
 This method requires some blurring of the layers in the gateways,
 since the ARP server and the Internet routing table would normally
 not have any contact.  In this respect, it is somewhat
 unsatisfactory.  Still, it is fairly easy to implement, and does not
 have significant performance costs.  One problem is that if the
 original gateway crashes, there is no way for the source host to
 choose an alternate route even if one exists; thus, a connection that
 might otherwise have been maintained will be broken.
 One should not confuse this method of "ARP-based subnetting" with the
 superficially similar use of ARP-based bridges.  ARP-based subnetting
 is based on the ability of a gateway to examine an IP address and
 deduce a route to the destination, based on explicit subnet topology.
 In other words, a small part of the routing decision has been moved
 from the source host into the gateway.  An ARP-based bridge, in
 contrast, must somehow locate each host without any assistance from a
 mapping between host address and topology.  Systems built out of
 ARP-based bridges should not be referred to as "subnetted".
 N.B.: the use of ARP-based subnetting is complicated by the use of
 broadcasts.  An ARP server [7] should never respond to a request
 whose target is a broadcast address.  Such a request can only come
 from a host that does not recognize the broadcast address as such,
 and so honoring it would almost certainly lead to a forwarding loop.
 If there are N such hosts on the physical network that do not
 recognize this address as a broadcast, then a packet sent with a
 Time-To-Live of T could potentially give rise to T**N spurious
 re-broadcasts.

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4. Case Studies

 In this section, we briefly sketch how subnets have been used by
 several organizations.
 4.1. Stanford University
    At Stanford, subnets were introduced initially for historical
    reasons.  Stanford had been using the Pup protocols [1] on a
    collection of several Experimental Ethernets [5] since 1979,
    several years before Internet protocols came into use.  There were
    a number of Pup gateways in service, and all hosts and gateways
    acquired and exchanged routing table information using a simple
    broadcast protocol.
    When the Internet Protocol was introduced, the decision was made
    to use an eight-bit wide subnet number; Internet subnet numbers
    were chosen to match the Pup network number of a given Ethernet,
    and the Pup host numbers (also eight bits) were used as the host
    field of the Internet address.
    The Pup-only gateways were then modified to forward Internet
    datagrams according to their Pup routing tables; they otherwise
    had no understanding of Internet packets and in fact did not
    adjust the Time-to-live field in the Internet header.  This seems
    to be acceptable, since bugs that caused forwarding loops have not
    appeared.  The Internet hosts that are multi-homed and thus can
    serve as gateways do adjust the Time-to-live field; since all of
    the currently also serve as Pup gateways, no additional routing
    information exchange protocol was needed.
    Internet host implementations were modified to understand subnets
    (in several different ways, but with identical effects).  Since
    all already had Pup implementations, the Internet routing tables
    were maintained by the same process that maintained the Pup
    routing tables, simply translating the Pup network numbers into
    Internet subnet numbers.
    When 10Mbit Ethernets were added, the gateways were modified to
    use the ARP-based scheme described in an earlier section; this
    allowed unmodified hosts to be used on the 10Mbit Ethernets.
    IP subnets have been in use since early 1982; currently, there are
    about 330 hosts, 18 subnets, and a similar number of subnet
    gateways in service.  Once the Pup-only gateways are converted to
    be true Internet gateways, an Internet-based routing exchange
    protocol will be introduced, and Pup will be phased out.

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 4.2. MIT
    MIT was the first IP site to accumulate a large collection of
    local network links.  Since this happened before network numbers
    were divided into classes, to have assigned each link at MIT its
    own IP network number would have used up a good portion of the
    available address space.  MIT decided to use one IP network
    number, and to manage the 24-bit "rest" field itself, by dividing
    it into three 8-bit fields; "subnet", "reserved, must be zero",
    and "host".   Since the CHAOS protocol already in use at MIT used
    an 8-bit subnet number field, it was possible to assign each link
    the same subnet number in both protocols.  The IP host field was
    set to 8 bits since most available local net hardware at that
    point used 8 bit addresses, as did the CHAOS protocol; it was felt
    that reserving some bits for the future was wise.
    The initial plan was to use a dynamic routing protocol between the
    IP subnet gateways; several such protocols have been mooted but
    nobody has bothered to implement one; static routing tables are
    still used.  It is likely that this change will finally be made
    soon.
    To solve the problem that imported IP software always needed
    modification to work in the subnetted environment, MIT searched
    for a model of operation that led to the least change in host IP
    software.  This led to a model where IP gateways send ICMP Host
    Redirects rather than Network Redirects.  All internal MIT IP
    gateways now do so.  With hosts that can maintain IP routing
    tables for non-local communication on a per host basis, this hides
    most of the subnet structure.  The "minimum adjustment" for host
    software to work correctly in both subnetted and non-subnetted
    environments is the bit-mask algorithm mentioned earlier.
    MIT has no immediate plans to move toward a single "approved"
    protocol; this is due partly to the degree of local autonomy and
    the amount of installed software, and partly to the lack of a
    single prominent industry standard.  Rather, the approach taken
    has been to provide a single set of physical links and packet
    switches, and to layer several "virtual" protocol nets atop the
    single set of links.  MIT has had some bad experiences with trying
    to exchange routing information between protocols and wrap one
    protocol in another; the general approach is to keep the protocols
    strictly separated except for sharing the basic hardware.  Using
    ARP to hide the subnet structure is not much in favor; it is felt
    that this overloads the address resolution operation.  In a
    complicated system (i.e. one with loops, and variant link speeds),

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    a more sophisticated information interchange will be needed
    between gateways; making this an explicit mechanism (but one
    insulated from the hosts) was felt to be best.
 4.3. Carnegie-Mellon University
    CMU uses a Class B network currently divided into 11 physical
    subnets (two 3Mbit Experimental Ethernets, seven 10Mbit Ethernets,
    and two ProNet rings.) Although host numbers are assigned so that
    all addresses with a given third octet will be on the same subnet
    (but not necessarily vice versa), this is essentially an
    administrative convenience.  No software currently knows the
    specifics of this allocation mechanism or depends on it to route
    between cables.
    Instead, an ARP-based bridge scheme is used.  When a host
    broadcasts an ARP request, all bridges which receive it cache the
    original protocol address mapping and then forward the request
    (after the appropriate adjustments) as an ARP broadcast request
    onto each of their other connected cables.  When a bridge receives
    a non-broadcast ARP reply with a target protocol address not its
    own, it consults its ARP cache to determine the cable onto which
    the reply should be forwarded.  The bridges thus attempt to
    transparently extend the ARP protocol into a heterogenous
    multi-cable environment.  They are therefore required to turn ARP
    broadcasts on a single cable into ARP broadcasts on all other
    connected cables even when they "know better".  This algorithm
    works only in the absence of cycles in the network connectivity
    graph (which is currently the case).  Work is underway to replace
    this simple-minded algorithm with a protocol implemented among the
    bridges, in support of redundant paths and to reduce the
    collective broadcast load.  The intent is to retain the ARP base
    and host transparency, if possible.
    Implementations supporting the 3Mbit Ethernet and 10Mb proNET ring
    at CMU use RFC-826 ARP (instead of some wired-in mapping such as
    simply using the 8-bit hardware address as the the fourth octet of
    the IP address).
    Since there are currently no redundant paths between cables, the
    issue of maintaining connections across bridge crashes is moot.
    With about 150 IP-capable hosts on the net, the bridge caches are
    still of reasonable size, and little bandwidth is devoted to ARP
    broadcast forwarding.
    CMU's network is likely to grow from its relatively small,
    singly-connected configuration centered within their CS/RI

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RFC 917 October 1984 Internet Subnets

    facility to a campus-wide intra-departmental configuration with
    5000-10000 hosts and redundant connections between cables.  It is
    possible that the ARP-based bridge scheme will not scale to this
    size, and a system of explicit subnets may be required.  The
    medium-term goal, however, is an environment into which unmodified
    extant (especially 10Mb ethernet based) IP implementations can be
    imported; the intent is to stay with a host-transparent (thus
    ARP-based) routing mechanism as long as possible.  CMU is
    concerned that even if subnets become part of the IP standard they
    will not be widely implemented; this is the major obstacle to
    their use at CMU.

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RFC 917 October 1984 Internet Subnets

I. Address Format ICMP

 Address Format Request or Address Format 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         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    IP Fields:
       Addresses
          The address of the source in an address format request
          message will be the destination of the address format reply
          message.  To form an address format 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 A2, the
          subnet field width inserted into the Code 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
          A1 for address format request message
          A2 for address format reply message
       Code
          0 for address format request message
          Width of subnet field, in bits, for address format reply
          message
       Checksum
          The checksum is the 16-bit one's complement of the one's

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RFC 917 October 1984 Internet Subnets

          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 request and replies, may be
          zero.
       Sequence Number
          A sequence number to aid in matching request and replies,
          may be zero.
    Description
       A gateway receiving an address format request should return it
       with the Code field set to the number of bits of Subnet number
       in IP addresses for the network to which the datagram was
       addressed.  If the request was broadcast, the destination
       network is "this network".  The Subnet field width may be from
       0 to (31 - N), where N is the width in bits of the IP net
       number field (i.e., 8, 16, or 24).
       If the requesting host does not know its own IP address, it may
       leave the source field zero; the reply should then be
       broadcast.  Since there is only one possible address format for
       a network, there is no need to match requests with replies.
       However, this approach should be avoided if at all possible,
       since it increases the superfluous broadcast load on the
       network.
          Type A1 may be received from a gateway or a host.
          Type A2 may be received from a gateway, or a host acting in
          lieu of a gateway.

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RFC 917 October 1984 Internet Subnets

II. Examples

 For these examples, we assume that the requesting host has  address
 36.40.0.123, that there is a gateway at 36.40.0.62, and that on
 network 36.0.0.0, an 8-bit wide subnet field is in use.
 First, suppose that broadcasting is allowed, and that 36.40.0.123
 knows  its own address.  It sends the following datagram:
    Source address:          36.40.0.123
    Destination address:     36.255.255.255
    Protocol:                ICMP = 1
    Type:                    Address Format Request = A1
    Code:                    0
 36.40.0.62 will hear the datagram, and should respond with this
 datagram:
    Source address:          36.40.0.62
    Destination address:     36.40.0.123
    Protocol:                ICMP = 1
    Type:                    Address Format Reply = A2
    Code:                    8
 For the following examples, assume that address 255.255.255.255
 denotes "broadcast to this physical network", as described in [6].
 The previous example is inefficient, because it potentially
 broadcasts  the request on many subnets.  The most efficient method,
 and the one we recommend, is for a host to first discover its own
 address (perhaps  using the "Reverse ARP" protocol described in [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 Format Request = A1
    Code:                    0
 The gateway can then respond directly to the requesting host.
 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:

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RFC 917 October 1984 Internet Subnets

    Source address:          0.0.0.0
    Destination address:     255.255.255.255
    Protocol:                ICMP = 1
    Type:                    Address Format Request = A1
    Code:                    0
 36.40.0.62 will hear the datagram, and should respond with this
 datagram:
    Source address:          36.40.0.62
    Destination address:     36.40.255.255
    Protocol:                ICMP = 1
    Type:                    Address Format Reply = A2
    Code:                    8
 Note that the gateway uses the narrowest possible broadcast to reply
 (i.e., sending the reply to 36.255.255.255 would mean that it is
 transmitted on many subnets, not just the one on which it is needed.)
 Even so, the overuse of broadcasts presents an unnecessary load to
 all hosts on the subnet, and so we recommend that use of the
 "anonymous" (0.0.0.0) source address 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 Format Request = A1
    Code:                    0
 36.40.0.62 should respond exactly as in the previous case.

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RFC 917 October 1984 Internet Subnets

Notes

 <1>  For example, some host have addresses assigned by concatenating
      their Class A network number with the low-order 24 bits of a
      48-bit Ethernet hardware address.
 <2>  Our discussion of Internet broadcasting is based on [6].
 <3>  If broadcasting is not supported, them presumably a host "knows"
      the address of a neighbor gateway, and should send the ICMP to
      that gateway.
 <4>  This is what was referred to earlier as the coexistence of
      transparent and explicit subnets on a single network.

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RFC 917 October 1984 Internet Subnets

References

 1.  D.R. Boggs, J.F. Shoch, E.A. Taft, and R.M. Metcalfe. "Pup: An
     Internetwork Architecture."  IEEE Transactions on Communications
     COM-28, 4, pp612-624, April 1980.
 2.  David D. Clark.  Names, Addresses, Ports, and Routes.  RFC-814,
     MIT-LCS, July 1982.
 3.  Yogan K. Dalal and Robert M. Metcalfe. "Reverse Path Forwarding
     of Broadcast Packets."  Comm. ACM 21, 12, pp1040-1048, December
     1978.
 4.  Ross Finlayson, Timothy Mann, Jeffrey Mogul, Marvin Theimer. A
     Reverse Address Resolution Protocol. RFC-903, Stanford
     University, June 1984.
 5.  R.M. Metcalfe and D.R. Boggs. "Ethernet: Distributed Packet
     Switching for Local Computer Networks."  Comm. ACM 19, 7,
     pp395-404, July 1976.  Also CSL-75-7, Xerox Palo Alto Research
     Center, reprinted in CSL-80-2.
 6.  Jeffrey Mogul. Broadcasting Internet Datagrams. RFC-919, Stanford
     University, October 1984.
 7.  David Plummer. An Ethernet Address Resolution Protocol. RFC-826,
     Symbolics, September 1982.
 8.  Jon Postel. Internet Protocol. RFC-791, USC-ISI, September 1981.
 9.  Jon Postel. Internet Control Message Protocol. RFC-792, USC-ISI,
     September 1981.

Mogul [Page 22]

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