As per Relevance of the word addresses, we have this rfc below:
Network Working Group David D.
Request for Comments: 932 MIT,
January 1985
A SUBNETWORK ADDRESSING
STATUS OF THIS
This RFC suggests a proposed protocol for the ARPA-
community, and requests discussion and suggestions for improvements
Distribution of this memo is unlimited
Several recent RFCs have discussed the need for a "subnet"
within the internet addressing scheme, and have proposed
for "subnetwork" addressing and routing. In particular, Jeff
in his RFC-917, "Internet Subnets", describes an addressing scheme
which a variable number of the leading bits of the host portion
the address are used to identify the subnet. The drawback to
scheme is that it is necessary to modify the host implementation
order to implement it. While the modification is a simple one, it
necessary to retrofit it into all implementations, including
which are already in the field. (See RFC-917 by Mogul for
alternative approaches to this problem, such as using
Resolution Protocol.)
This RFC proposes an alternative addressing scheme for subnets which
in most cases, requires no modification to host software whatsoever
The drawbacks of this scheme are that the total number of subnets
any one network are limited, and that modification is required to
gateways
THE
In this scheme, the individual subnets of a network are
using Class C addresses. Since it is necessary with this scheme
a Class C address used to number a subnet be distinguishable from
Class C address used to number an isolated network, we will
for subnetworks the upper half of the Class C address space, in
words all those Class C addresses for which the high order bit is on
When a network is to be organized as a series of subnetworks, a
of these reserved Class C addresses will be assigned to that network
specifically a block of 256 addresses having the two first
identical. Thus, the various subnetworks of a network
distinguished by the third byte of the Internet address. (
addressing scheme implies the limitation that there can only be 256
subnetworks in a net. If more networks are required, two blocks
have to be allocated, and the total viewed as two separate networks.)
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RFC 932 January 1985
A Subnetwork Addressing
The gateways and hosts attached to this subnetted network use
addresses as ordinary Class C addresses. Thus, no modification
any host software is required for hosts attached to a subnetwork
For gateways not directly attached to the subnetted network, it is
unacceptable burden to separately store the routing information
each of the subnets. The goal of any subnet addressing scheme is
provide a strategy by which distant gateways can store
information for the network as a whole. In this scheme, since
first two bytes of the address is the same for every subnet in
network, those first two bytes can be stored and manipulated as
they are a single Class B address by a distant gateway.
addresses, which can be used either as a Class B or Class C
as appropriate, have been informally called Class "B 1/2" addresses
In more detail, a gateway would treat Class C addresses as
under the scheme. First, test to see whether the high order bit
the address is on. If not, the address is an ordinary Class
address and should be treated as such
If the bit is on, this Class C address identifies a subnet of
network. Test to see if this gateway is attached to that network
If so, treat the address as an ordinary Class C address
If the gateway is not attached to the network containing
subnetwork, discard the third byte of the Class C address and
the resulting two bytes as a Class B address. Note that there can
no conflict between this two-byte pattern and an ordinary Class
address, because the first bits of this address are not those of
valid Class B address, but rather those of a Class C address
If a network grows to more than 256 subnetworks, it will be
to design two distinct blocks of special Class C addresses, and
view this aggregate as two separate networks. However, the
of these two networks can, by proper design, run a joint
algorithm which maintains optimal routes between the two halves,
if they are connected together by a number of gateways
Indeed, in general it is possible for gateways that are not
attached to a subnetworked network to be specially programmed
remember the individual Class C addresses, if doing so
greatly improved network efficiency in some particular case
It was stated earlier that no modification to the host software
necessary to implement this scheme. There is one case in which
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RFC 932 January 1985
A Subnetwork Addressing
minor modification may prove helpful. Consider the case of a
host, not immediately attached to this subnetworked network.
host, even though at a distance, will nonetheless maintain
routing entries for each of the distinct subnetwork addresses
which it has any knowledge. For most hosts, storing this
for each subnet represents no problem, because most
do not try to remember routing information about every
address in the Internet, but only those addresses that are of
interest. If, however, for some reason the host has a table
attempts to remember routing information about every Internet
it has ever seen, than that host should be programmed to
the gateway's algorithm for collapsing the addresses of
subnets from three bytes to two. However, it is not a
implementation strategy for the host to maintain this degree
routing information, so under normal circumstances, the host need
be concerned with the C to B conversion
The major drawback of this scheme is that any implementation
large tables of addresses must be changed to know the "B 1/2"
conversion rule. Most importantly, all gateways must be programmed
know this rule. Thus, adoption of this scheme will require
scheduled mandatory change by every gateway implementation.
difficulty of organizing this is unknown
OTHER
It is possible to imagine other variations on the patterns
collapsing addresses. For example, 256 Class B addresses could
gathered together and collapsed into one Class A address. However
since the first three bits of the resulting Class A address would
constrained, this would permit only 32 such subnetted networks
exist. A more interesting alternative would be to permit
collapse of Class C addresses into a single Class A address. It
not entirely obvious the best way of organizing the sub-fields
this address, but this combination would permit a few very large
of subnets to be assembled within the Internet
The most interesting variation of "B 1/2" addresses is to
the number of bits used to identify the subnet by taking bits
the resulting Class B address. For example, if 10 bits were used
identify the subnet (providing 1024 subnets per network), then
gateway, when forming the equivalent address, would not only drop
third byte but also mask the last two bits of the B address.
the first three bits of the address are constrained, this would
13 bits for the network number, or 8192 possible
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RFC 932 January 1985
A Subnetwork Addressing
networks. This number is not as large as would be desirable, so
is clear that selecting the size of the subnet field is an
compromise
Danny Cohen has suggested that this scheme should be
generalized so that the boundaries between the network, subnetwork
and host field be arbitrarily movable. The problem in such
generalization is to determine how the gateway is to maintain
table or algorithm which permits the collapsing of the address
occur. This RFC proposes that, in the short run, only one
form of "B 1/2" addresses be implemented as an Internet
standard
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if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
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