Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!natinst!dell!baldur!dcm From: dcm@baldur.dell.com (Dave McCracken) Newsgroups: comp.protocols.tcp-ip Subject: How to set up subnets where logical subnet != physical subnet Keywords: subnet routing Message-ID: Date: 9 Apr 91 18:01:11 GMT Sender: @dell.dell.com Organization: Dell Computer Corporation, Austin, Texas 78759-7299 Lines: 41 I am trying to solve a problem we are having trying to set up a subnet in our corporate network. The current environment is an ethernet with MAC-layer bridges between all subnets, which are necessary because we need to support IP, Netware IPX, and 3com 3+share packets. What we would like to do is split off the Unix development group as an IP-only subnet, with the rest of the network as the other subnet. What we would end up with is a small subnet with 30-40 machines connected to a large subnet with 500-800 machines, of which probably 100-300 will use IP at least some of the time. Our IP address numbering scheme is a class B network address, and we would like to run with a 6-8 bit subnet mask. We have already assigned subnet numbers based on department for administrative reasons. The real problem is when I try to set up a router for this arrangement. Since most of the network is being routed by a layer below IP, it is necessary for several logical subnets, based on the subnet mask, to be on the same physical network. The routing code in the IP driver will cheerfully accept that the other subnets are local when I specify 0 hops to the route command, but it absolutely refuses to let me specify an IP address for the router that is not in the same logical subnet. We are currently running mostly System V Release 4, but the same problem exists on our Suns and in the straight BSD4.3 code (I looked in the source). What I would like to know from the collected wisdom of Usenet is why the restriction is there, and if you think anything would break if I changed the IP driver in SVR4 to accept a router address outside the subnet. I would also like to know is there is a simple way in the router to present miltiple IP addresses without plugging in extra network cards. This would be an alternate solution that would not require changing all clients. Thanks, -- Dave McCracken dcm@dell.dell.com (512) 343-3720 Dell Computer 9505 Arboretum Blvd Austin, TX 78759-7299