As per Relevance of the word addresses, we have this rfc below:
Network Working Group Michael J.
Request for Comments: 936 UC
February 1985
Another Internet Subnet 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
There have been several proposals for schemes to allow the use of
single Internet network number to refer to a collection of
networks under common administration which are reachable from
rest of the Internet by a common route. Such schemes allow
simplified view of an otherwise complicated topology from hosts
gateways outside of this collection. They allow the complexity
the number and type of these networks, and routing to them, to
localized. Additions and changes in configuration thus cause
detectable change, and no interruption of service, due to
propagation of routing and other information outside of the
environment. These schemes also simplify the administration of
network, as changes do not require allocation of new network
for each new cable installed. The motivation for explicit
implicit subnets, several of the alternatives, and descriptions
existing implementations of this type have been described in
[1,2]. This proposal discusses an alternative scheme, one that
been in use at the University of California, Berkeley
April 1984.
Subnet Addressing at
As in the proposal by Jeff Mogul in RFC-917, the Berkeley
addressing utilizes encoding of the host part of the
address. Hosts and gateways on the local network are able
determine the subnet number from each local address, and then
local packets based on the subnet number. Logically, the
of subnets appears to external sites to be a single,
network. Internally, however, each subnet is distinguished from
others and from other networks, and internal routing decisions
based on the subnet rather than the network number
The encoding of subnet addresses is similar to that proposed
RFC-917. In decomposing an Internet address into the network
host parts, the algorithm is modified if the network is "local",
is, if the network is a directly-connected network under
administrative control. (Networks are marked as local or non-
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Another Internet Subnet Addressing
at the time each network interface's address is set at boot time.)
For local addresses, the host part is examined for a subnet number
Local addresses may be on the main network, or they may be on
subnet. The high-order bit of the host number is used to
between subnets and the main net. If the high-order bit of the
field is set, then the remainder of the high-order byte of the
part is taken to be the subnet number. If the high-order bit
clear, then the address is interpreted in the normal fashion.
Class A networks, using 8-bit subnet fields, this allows a
with up to 127 subnets, each of 65535 hosts maximum, and a main
with 2^23 hosts. Class B nets may include 127 subnets, each of up
255 hosts, and 32767 hosts on the main net. Class C networks are
currently included in this scheme. They might be reasonably be added
using four bits of the host part for a subnet desgination and
bits for the host, allowing 8 subnets of 15 hosts and 126 hosts
the main net
The current implementation does not use subnet numbers
from the network field, but instead treats the subnet field as
extension of the network field. Functions that previously
the network number from an address now return a network
network-subnetwork number. Conveniently, Class A subnets
distinguishable from Class B networks, although each is a 16-
quantity, and Class B subnets are disjoint with Class C
numbers. The net result is that subnets appear to be separate
independent networks with their own routing entries within
network, but outside of the network, they are invisible. There is
current facility at Berkeley for broadcasting on the logical network
broadcasting may be done on each subnet that uses harware capable
broadcast
There have been several earlier proposals for methods of
several physical networks to share an Internet network designation
and to provide routing within this logical network. RFC-917
a means for encoding the host part of each local address such
the hosts, or the gateways connecting them, are able to determine
physical network for the host. The current proposal is most
to that scheme; the differences are discussed in detail below
Another proposal (RFC-925) involves the use of intelligent
to perform routing for unmodified hosts, using an Address
Protocol (ARP) [2]. This has the advantage of placing
modifications in the gateways, but is likely to require
routing protocols and caching mechanisms in the gateways in order
avoid excessive broadcasts for address resolution. A modification
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RFC 936 February 1985
Another Internet Subnet Addressing
this method is to perform encoding of subnets within host
by convention to simplify the routing in the gateways,
modifying host software to recognize these subnet addresses.
techniques were not considered for use at Berkeley, because
packet forwarding was being done by multi- homed hosts, all of
ran the same software as the singly-homed hosts (4.2BSD Unix).
The most recent proposal, RFC-932 [3], provides subnetting
encoding the network part of the Internet address rather than
host part. Ordinary hosts need not know of this convention
eliminating the need for modification to host software.
would be able to take advantage of this encoding to compress
routing information for the collection of networks into a
entry. Unfortunately, implementation of that scheme would require
fairly concerted transition by the gateways of the Internet, or
transition period would be likely to overflow the routing tables
the existing gateways. All of the hosts on the larger networks
be forced to change addresses from their current Class A or
addresses to "B 1/2" addresses. There are a limited number (4096)
blocks of Class C addresses available using this encoding.
number of universities and other organizations having
implemented subnets or contemplating their installation argues for
more extensible scheme, as well as one that can be implemented
quickly
The current proposal is most similar to that of RFC-917; indeed,
two implementations are nearly compatible. There are two
of significance. First, the use of a bit to distinguish
addresses from non-subnetted addresses allows both smaller
and a larger (physical or logical) main network. Half of the
addresses within a Class A or B network are reserved for use
subnets, the other half are available for the primary net. This
useful when using a hardware medium that is capable of
large numbers of hosts or for transparent subnetting (e.g.
ARP-based bridges). The corresponding disadvantage is that
subnets may be supported. The allocation of bits between the
number and the host field could be adjusted, but for Class
networks, neither is excessively large. Given the limited
space of the current Internet addressing, this is a difficult choice
The second difference is that the width of the subnet field is
in advance. This simplifies the already-too-complicated code
interpret Internet addresses, and avoids the bootstrap problem.
the subnet field width is to be determined dynamically, some
of the hosts on a network must be prepared to specify this value,
the situation will be unworkable if one of these hosts does not
the correct choice or none are accessible when other machines
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Another Internet Subnet Addressing
up. Also, the recovery procedure proposed by RFC-917
unnecessarily complicated and liable to fail. Dynamic discovery
this value depends on another modification as well, the addition of
new ICMP request. The alternatives are to specify the field size
a standard, or to require each implementation to be configurable
advance (e.g with a system compilation option or the use of a
patch installed when a host is initially installed. The use of
standard field width seems preferable, and an 8-bit field allows
most efficient implementations on most architectures. For Class
nets, a 4-bit field seems the only choice for a standard division
[1] J. Mogul, "Internet Subnets", RFC-917, Stanford University
October 1984
[2] J. Postel, "Multi-LAN Address Resolution", RFC-925, USC-ISI
October 1984
[3] D. Clark, "A Subnet Addressing Scheme", RFC-932, MIT-LCS
January 1985
Karels [Page 4]
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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