Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!VENERA.ISI.EDU!braden From: braden@VENERA.ISI.EDU Newsgroups: comp.protocols.tcp-ip Subject: Re: Subnetting Message-ID: <8805091611.AA07124@braden.isi.edu> Date: 9 May 88 16:11:40 GMT Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 45 > > 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