Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!julius.cs.uiuc.edu!apple!olivea!orc!bu.edu!buit13!kwe From: kwe@buit13.bu.edu (Kent England) Newsgroups: comp.protocols.tcp-ip Subject: Re: Multiple subnets on same cable Summary: Hosts and routers can see it differently Message-ID: <68286@bu.edu.bu.edu> Date: 9 Nov 90 18:33:37 GMT References: Sender: news@bu.edu.bu.edu Reply-To: kwe@bu.edu (Kent England) Followup-To: comp.protocols.tcp-ip Organization: Boston U. Information Technology Lines: 19 >What happens if different IP subnets live on the same ethernet cable? This question comes up often and bears mention occasionally. Don't forget that what happens with subnets depends on who is looking and how they are configured. If you subnet your (eg) Class B with eight bit subnet masks and you don't want (can't have) variable length subnets, then subnet layering looks attractive for configuring your routers. But your hosts can see things differently than their routers. If you layer subnets, you may set the subnet mask on these layered subnet hosts to be the network mask. They will then ARP for all members of their (subnetted) net and communicate directly to all their directly attached neighbors with no extra hops, failure points or extra traffic and the routers will answer for off-net members of the subnetted net, if your routers are set to proxy ARP. It works for 4.2BSD and it works well for layered subnets.