Re: Subnetting


braden@venera.isi.edu
Mon, 9 May 88 09:11:40 PDT


> > 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.

Stuart,

Let's consider the three examples you cite.

Network Redirect: It is recognized that network redirects are a problem
in a subnetted environment, and therefore the Gateway Specification
RFC-1009 says that gateways should only be sending host redirects.
If you have a gateway within your subnetted environment that is sending
network redirects, it should be brought up to spec.

Synthesizing a remote broadcast -- presumably you mean a directed
broadcast. Yes, this is a real engineering problem, although it is an
application-level problem, not an IP level problem. If you have an
application that is sending directed broadcasts into another subnet of
the same network, that application needs some configuration information
-- obviously, it needs the remote subnet number. You might as well
configure it with the complete 32-bit Internet directed broadcast
address. Synthesizing IP addresses should be avoided whenever possible.

Routing to a remote subnet -- This is entirely a gateway problem.
The solution, as RFC-1009 suggests, is simply to include the
subnet masks with the network numbers in the routing updates among
the subnet gateways. I wonder why no one has extended RIP in this
obvious way yet. As you say, it is merely a matter of a little
engineering.

After all this, I would challenge your statement that "in general,
you -do- have to be able to know the sizes of sibling subnets"
(at least, if "you" is a host, not a gateway).

Bob Braden



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