Re: Subnetting


Stuart Levy (slevy@uc.msc.umn.edu)
Mon, 9 May 88 02:27:22 CDT


> > b) "Subs" of a given "whole" must be of equal size.
>
> This is a mistaken assumption. There is nothing that prevents you
> from using subnets of different sizes on a given net, except for
> software that isn't up to speed on subnetting (notably SunOS 3.x).

Unfortunately, there -are- problems with dividing a network into
variable-sized subnets -- not just incomplete software implementations
but real engineering problems. They relate to cases where hosts or
gateways need to know the size of a subnet they're not attached to:
e.g. when interpreting an ICMP network redirect, synthesizing a remote
broadcast address, or routing to a remote subnet.

You may be able to live with the effects of misinterpretations if things
are kept simple enough (so long as nothing sends you a network redirect :->).

The subnetting RFCs, e.g. 917, 936, 950 discuss some possible conventions
for determining subnet sizes, including equal sizes on a given network (easy)
and self-encoding subnet sizes analogous to the class A/B/C sizing for ordinary
networks. I suppose it would also be possible to distribute a table of
subnet sizes to every host and gateway on a network. But in general,
you -do- have to be able to know the sizes of sibling subnets, and the
equal-size case seems to be the closest one to a standard.

        Stuart Levy



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