Username / Password : Request For Comments

RFC Number : 917

Title : Internet subnets.

Network Working Group Jeffrey Mogul
Request for Comments: 917 Computer Science Department
Stanford University
October 1984


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.


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.


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

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

Mogul [Page 1]

RFC 917 October 1984
Internet Subnets

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:

- 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 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

- 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

Mogul [Page 2]

RFC 917 October 1984
Internet Subnets

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

The rest of this document describes approaches to subnets of Internet

Mogul [Page 3]

RFC 917 October 1984
Internet Subnets

1.1. Terminology

To avoid either ambiguity or prolixity, we will define a few
terms, which will be used in the following sections:


The collection of connected Internet Networks


A single Internet network (that may or may not be divided into


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

Host Field

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


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

Mogul [Page 4]

RFC 917 October 1984
Internet Subnets


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

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

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

Mogul [Page 5]

RFC 917 October 1984
Internet Subnets

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

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

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:

where the field is as in [8], the
field is at least one bit wide, and the width of the
field is constant for a given network. No further
structure is required for the or
fields. If the width of the field is zero, then
the network is not subnetted (i.e., the interpretation of [8] is

Mogul [Page 6]

RFC 917 October 1984
Internet Subnets

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:

- There is no obvious need for a four-level hierarchy.

- The number of bits available in an IP address is not large
enough to make this useful in general.

- 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)
send_packet_locally(packet, 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

The code then becomes:

Mogul [Page 7]

RFC 917 October 1984
Internet Subnets

IF bitwise_and(packet.ip_dest, my_ip_mask)
= bitwise_and(my_ip_addr, my_ip_mask)
send_packet_locally(packet, packet.ip_dest)
gateway_to(bitwise_and(packet.ip_dest, my_ip_mask)))

Of course, part of the expression in the conditionally can be

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 ( causes the
a datagram to be sent as a broadcast on the local physical
network; it must not be forwarded by any gateway.

Mogul [Page 8]

RFC 917 October 1984
Internet Subnets

Specific network

The destination address contains a valid network number; the
local address part is all ones (e.g.,

Specific subnet

The destination address contains a valid network number and a
valid subnet number; the host field is all ones (e.g.,

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

'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,

Mogul [Page 9]

RFC 917 October 1984
Internet Subnets

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

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

2. Subnets are not in use, and no host supports this ICMP

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.

Mogul [Page 10]

RFC 917 October 1984
Internet Subnets

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:

Mogul [Page 11]

RFC 917 October 1984
Internet Subnets

- Be used only on singly-homed hosts, and not as a gateway.

- Be used on a broadcast LAN.

- Use an Address Resolution Protocol (ARP), such [7].

- Not be required to maintain connections in the case of gateway

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

Mogul [Page 12]

RFC 917 October 1984
Internet Subnets

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.

Mogul [Page 13]

RFC 917 October 1984
Internet Subnets

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

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),

Mogul [Page 14]

RFC 917 October 1984
Internet Subnets

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

Mogul [Page 15]

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.

Mogul [Page 16]

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:


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:


A1 for address format request message

A2 for address format reply message


0 for address format request message

Width of subnet field, in bits, for address format reply


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

Mogul [Page 17]

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.


An identifier to aid in matching request and replies, may be

Sequence Number

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


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

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.

Mogul [Page 18]

RFC 917 October 1984
Internet Subnets

II. Examples

For these examples, we assume that the requesting host has address, that there is a gateway at, and that on
network, an 8-bit wide subnet field is in use.

First, suppose that broadcasting is allowed, and that
knows its own address. It sends the following datagram:

Source address:
Destination address:
Protocol: ICMP = 1
Type: Address Format Request = A1
Code: 0 will hear the datagram, and should respond with this

Source address:
Destination address:
Protocol: ICMP = 1
Type: Address Format Reply = A2
Code: 8

For the following examples, assume that address
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

Source address:
Destination address:
Protocol: ICMP = 1
Type: Address Format Request = A1
Code: 0

The gateway can then respond directly to the requesting host.

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

Mogul [Page 19]

RFC 917 October 1984
Internet Subnets

Source address:
Destination address:
Protocol: ICMP = 1
Type: Address Format Request = A1
Code: 0 will hear the datagram, and should respond with this

Source address:
Destination address:
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 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' ( source address be kept to a minimum.

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

Source address:
Destination address:
Protocol: ICMP = 1
Type: Address Format Request = A1
Code: 0 should respond exactly as in the previous case.

Mogul [Page 20]

RFC 917 October 1984
Internet Subnets


<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.

Mogul [Page 21]

RFC 917 October 1984
Internet Subnets


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

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]

Site Hosted By Digital Environments, Inc. This Website was Created with DE-Web Version,
The Fast, Web Based - Website Design Tool, Groupware and Web Hosting System by Digital Environments, Inc.
Groupware:Project Management, Sales Tracking, Web Site Design and News / Blogger all in one package.