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From: BILLW@mathom.cisco.com (WilliamChops Westfield)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: Serial interfaces IP number question
Message-ID: <24823@boulder.Colorado.EDU>
Date: 15 Aug 90 23:32:23 GMT
Sender: daemon@boulder.Colorado.EDU
Lines: 45

> We have the class B 131.195 address; we want to interconnect
> 3 different sites with 3 different subnets, what should I use for
> serial interfaces IP numbers? Each of my 3 Cisco have one ethernet and two 
> serial interfaces and we want to make a triangle with 3 Cisco's and 
> 3 serial links.
> 
> I don't want to waist subnet because we are planning to interconnect
> about 100 sites eventually.
> 
> We use a 9 bits subnet (max. of 128 subnets with 512 machines each);
> 	Site A = 131.195.2.x     We have about 400 machines per site
> 	Site B = 131.195.4.x
> 	Site C = 131.195.6.x

Well, I really like the idea I heard someone suggest a while ago on some
other list - assign the subnet bits high-bit first, and the host addresses
low bits first.  This allows you to easilly expand the subnet field as
the number of nets you are actually using goes up.

For example, consider a class B address N.N.0.0.  If you only have 3
networks now, you can get by with a 3 bit subnet mask (255.255.224.0),
and assign 6 networks:

	N.N.128.0
	N.N.64.0
	N.N.192.0
	N.N.32.0
	N.N.96.0
	N.N.160.0

If you need to add more networks, you change the netmask to 4 bits
(255.255.240.0), freeing up an additional 8 networks, but all the old
network numbers are still valid.

This way, you only run into a problem if you actually get too many
networks with too many hosts on each one.

With the 8.1 software, you need not allocate a subnet to serial links,
which may help in your case.  Also, if only a few of the networks have
400 hosts, you can set up the ciscos to put two subnets worth of hosts
on a single cable.

Bill Westfield
cisco Systems.
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