Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!rochester!cornell!gvax!jqj From: jqj@gvax.cs.cornell.edu (J Q Johnson) Newsgroups: comp.protocols.tcp-ip Subject: Re: How do you break up a B class number? Message-ID: <940@gvax.cs.cornell.edu> Date: Thu, 23-Jul-87 07:05:44 EDT Article-I.D.: gvax.940 Posted: Thu Jul 23 07:05:44 1987 Date-Received: Sat, 25-Jul-87 05:36:44 EDT References: <11636@hi.UUCP> Reply-To: jqj@gvax.cs.cornell.edu (J Q Johnson) Organization: Cornell Univ. CS Dept, Ithaca NY Lines: 24 In article <11636@hi.UUCP> kurt@hc.dspo.gov (Kurt Zeilenga) writes: >Does anyone know of any good references for netmasking (subnets) >schemes to split a B class number into various sized networks? Although variable-sized subnets may work for some network topologies, they are almost guaranteed to get you into trouble, and I strongly recommend that you avoid them. Consider a subnet (perhaps the backbone) with two or more gateways. Host A on this subnet wants to send a packet to host B on a different subnet. In order to look up the route to B, A needs to decompose B's address into net-subnet-host, so he needs to know B's subnet mask. All current software that I know of will use A's mask, and ASSUME that it is the same size as B's. Granted you can fool the routing tables in some topologies, e.g. a network with subnet mask of 255.255.255.0 containing several subsubnets (who think the subnet mask is 255.255.255.188 or something) all connected to only a single (hacked) gateway, where that gateway advertises a subnet that is the union of the subsubnets. It will break as soon as you make the topology more complex! Given that we can't do what Zeilenga asks, is it perhaps time to rethink the whole subnet scheme? 16 bits of hostnumber is not much at all for a typical large organization (say a university), especially if we have to waste most of it because of subnet constraints.