From owner-freebsd-questions  Fri May 11 10:31:32 2001
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To: Wayne Pascoe <wayne.pascoe@realtime.co.uk>
Cc: questions@FreeBSD.ORG
Subject: Re: OT: TCP/IP Subnetting 
In-reply-to: Your message of "11 May 2001 11:15:31 BST."
             <86k83oi43g.fsf@pan.realtime.co.uk> 
Date: Fri, 11 May 2001 10:31:27 -0700
From: "Kevin Oberman" <oberman@es.net>
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Wayne,

There are better possibilities.

Break up the /25 as follows:
Size   Addresses       Start Address    Net Mask
/26   62 addresses     128.1.1.128	255.255.255.192
/27   30 addresses     128.1.1.192	255.255.255.224
/28   14 addresses     128.1.1.224      255.255.255.240
/29    6 addresses     128.1.1.240      255.255.255.248

You may move the blocks around, but be careful calculating the
addresses!

Use the /29 for your 4 machine space. Use the other spaces for the
rest of the systems, starting with the largest (/26). You can work
communication by either setting up a system as a router between the
address spaces or, more cleanly, you can set up appropriate routing
table entries on each system with routes to the local network for each
subnet that is used in the LAN.

This means pointing 128.1.1.128, 128.1.1.192 and 128.1.1.224 at the
local link. See the route(8) and netstat(1) man pages for more hints
on how this can be done. Note that route(8) in FreeBSD does support
CIDR add/len notation to make this easier.

It has a major downside in requiring the configuration be loaded on
EVERY system.

While this looks ugly, it's how the Internet works and all providers
do this routinely, although it's far easier to configure on a Cisco or
Juniper than on a FreeBSD host.

R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman@es.net			Phone: +1 510 486-8634

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