Xref: utzoo comp.dcom.lans:1337 comp.protocols.tcp-ip:3422 Path: utzoo!mnetor!uunet!lll-winken!lll-tis!mordor!lll-lcc!unisoft!mtxinu!ed From: ed@mtxinu.UUCP (Ed Gould) Newsgroups: comp.dcom.lans,comp.protocols.tcp-ip Subject: Re: Subnetting Message-ID: <596@mtxinu.UUCP> Date: 9 May 88 18:37:55 GMT References: <358@halley.UUCP> <203@tekbspa.UUCP> Reply-To: ed@mtxinu.UUCP (Ed Gould) Organization: mt Xinu, Berkeley Lines: 48 >Now, the classes of IP addresses is simple enough to understand -- > >But what about subnetting? When does one want to *really* use >two IP network address on the same cable? And what performance >advantages does this give you? Were does the netmask come it at? The purpose of subnetting is not to run multiple IP addresses on the same cable, but to make a local collection of networks appear as if it were one network from the outside. To do this, one takes a standard IP adress (ususally a Class B address, but this isn't required) and uses some of the bits that are normally the "host number" part of the address as if they were part of the "network number." The netmask determines the division between host part and network part that is used locally. For example, consider the following /etc/hosts excerpt, which lists several Class B addresses. Keep in mind a netmask of 0xFFFFFF00 (the normal Class B netmask is 0xFFFF0000). 129.1.1.1 main-sys 129.1.2.1 main-sys-gw 129.1.2.2 second-sys 129.1.2.3 third-sys One topology this could represent is net to outside internal net (129.1.1) (129.1.2) ________ _________________________________ | | | | | | | | ----------- ------------- -------------- | | | | | | | main-sys| | second-sys| | third-sys | | | | | | | ----------- ------------- -------------- In this case, there are two physical networks: one connecting the three machines locally, and one connecting the main system to the outside world. To the outside world, the three machines look as if they are connected together on a single Class B network. Internally, though, they look as if they were on two separate Class C networks with a gateway. -- Ed Gould mt Xinu, 2560 Ninth St., Berkeley, CA 94710 USA {ucbvax,uunet}!mtxinu!ed +1 415 644 0146 "I'll fight them as a woman, not a lady. I'll fight them as an engineer."