As per Relevance of the word subnetting, we have this rfc below:
Network Working Group
Request for Comments: 940
April 1985
Toward an Internet Standard Scheme for
STATUS OF THIS
This RFC discusses standardizing the protocol used in
environments in the ARPA-Internet. Distribution of this memo
unlimited
The author of this RFC is the Gateway Algorithms and Data
(GADS) Task Force, chaired by David L. Mills
Several sites now contain a complex of local links connected to
Internet via a gateway. The details of the internal connectivity
of little interest to the rest of the Internet
One way of organizing these local complexes of links is to use
same strategy as the Internet uses to organize networks, that is,
declare each link to be an entity (like a network) and
interconnect the links with devices that perform routing
(like gateways). This general scheme is called subnetting,
individual links are called subnets, and the connecting devices
called subgateways (or bridges, or gateways).
All hosts in the Internet must make a decision when sending
datagram, that is, they must answer the question "Is this
addressed to a host on a directly connected network, or must it
sent to a gateway?". In a subnetted environment, this question
extended to "Is this datagram addressed to a host on a
connected subnet, or must it be sent to a (sub)gateway?". Let
call answering this question "making the routing decision".
Because the hosts used in a subnetted environment must implement
their IP or network interface software procedures for making
routing decision, and because such hosts may be acquired from
sources, it is important that a standard subnetting scheme
identified so that different suppliers can provide compatible
(that is, hosts compatible with the complexes at different sites
each other). Without a designated standard for a subnetting
suppliers can not create compatible hosts
The potential problem is that if different subnetting schemes
developed by different suppliers a customer that installs hosts
two or more suppliers may find that they do not work together
GADS [Page 1]
RFC 940 April 1985
Toward an Internet Standard Scheme for
This topic has been discussed in a set of RFCs [1,2,3,4] and in
flurry of messages in the Gateway Algorithms and Data Structures
Force. It is strongly suggested that if subnetting is used at all
it be according this new standard scheme
An Internet address currently consists of a two-layer hierarchy,
'network' and a per-network 'rest' field. This subnet scheme adds
optional 'subnet' layer and field
The subnet field is created by stealing some bits from the rest (
host) field of the address. The details of the subnet field are
specific. All three classes (A, B, and C) of networks may
subnetted
The use of subnets is an optional local decision. The fact that
network has subnets is invisible outside that network, and the
is local and can be instituted at a site without any global
perturbations. A complex of links is assigned a single IP
number, and outside that complex it appears as a single network
that number. Only inside does local structure appear
However, while the decision to use subnets at a site is optional,
IP implementation which may possibly be used in a
subnetted environment, should provide for subnet field
as described above. Such an implementation will function properly
environments with or without subnetting. On the other hand
implementations lacking this provision will not function in
subnetted environment, and are thus potentially less useful
This specifications is not intended to require a
implementation technique inside the host, but rather to define
external behavior of the host in a subnetted environment. It
not specify how routing is done or the details of host construction
Note that gateways are hosts, too
However, it seems easiest to explain the approach by describing
possible host implementation
Example Implementation
Let us use "subnet" to mean the locally attached
medium
The key decision to be made is "Is the destination IP
GADS [Page 2]
RFC 940 April 1985
Toward an Internet Standard Scheme for
on my subnet or not?". Once this decision is made the
knows to whether to send the datagram directly to
destination on the subnet or to send the datagram to a gateway
The host uses a 32-bit mask, along with the host's own
address, to determine whether or not destination IP
are on its subnet
The mask can be configured at boot time as a static quantity
distributed by a protocol that is beyond the scope of
memo
If the bitwise AND of the mask with the destination IP
matches the bitwise AND of the mask with the host's own
address, the destination is assumed on its subnet; if not,
destination is assumed on a subnet or network reachable
via a gateway
Note: if the mask is all zeros, all destinations will
to be on this subnet; while, if the mask is all ones,
the sending host itself will appear to be on this subnet
If the mask contains ones in the network field and zeros
the rest field, subnets are not in use
The above procedure must be treated as a per
procedure for multihomed hosts
For further information on background and rationale, see RFC-917,
"Internet Subnets" [1].
[1] Mogul, J., "Internet Subnets", RFC-917, Stanford University
October 1984.
[2] Postel, J., "Multi-LAN Address Resolution", RFC-925,
USC/Information Sciences Institute, October 1984.
[3] Clark, D., "A Subnetwork Addressing Scheme", RFC-932, MIT LCS
January 1985.
[4] Karels, M., "Another Internet Subnet Addressing Scheme",
RFC-936, UC Berkeley, February 1985.
GADS [Page 3]
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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