Re: Subnetting


Doug_Nelson@um.cc.umich.edu
Mon, 9 May 88 12:32:08 EDT


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Date: 9 May 88 18:37:55 GMT
From: mtxinu!ed@ucbvax.Berkeley.EDU (Ed Gould)
Organization: mt Xinu, Berkeley
Subject: Re: Subnetting
Message-Id: <596@mtxinu.UUCP>
References: <358@halley.UUCP>, <203@tekbspa.UUCP>
Sender: tcp-ip-request@sri-nic.arpa
To: tcp-ip@sri-nic.arpa

>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."



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