[nsp] /30 over WAN links

Voll, Scott Scott.Voll at wesd.org
Fri Feb 6 10:56:20 EST 2004

A /30 subnet allows for less waste of IP addresses.  A /30 subnet allows
for the network number, two usable IP addresses and a broadcast number.
A /29 subnet gives you a network number, broadcast and 6 IP addresses.  

Even with Private addressing we use /30 subnets for a point to point T1.
At one time when we had multiple FR t1's coming into one T1, and had not
used subinterfaces, we would use a /29 because all the routers were in
the same subnet.  Not best practice.  We changed that shortly after I

As far as other benefits, I can't think of any.  It's just a matter of
not using up IP's if you don't have too.  

It sounds like this other company is giving the customer there IP
addresses out of there WAN subnet.  Basically there shouldn't be
anything wrong with that.  We just happen to use on network range for
WAN stuff and give the customer a different subnet for there use.

Just my .02 worth.


-----Original Message-----
From: Steve Lim [mailto:limmer at execpc.com] 
Sent: Friday, February 06, 2004 7:33 AM
To: cisco-nsp at puck.nether.net
Subject: [nsp] /30 over WAN links

Hello all,

	It has been an age old policy at my company to place a /30 over
the WAN 
links ever since who knows when, and I've never really questioned it. 
But now, we've merged with another company, and they do not follow such 
a policy at the Access Layer. In fact, they use a /29 (or shorter 
prefixes, if customer requires more IPs) over the WAN links, and  use 
the IPs not already used by the respective end interfaces for 
hosts/devices on the remote/customer end.

	I had assumed that most, if not all companies use /30s. So this
came as 
a surprise. But more importantly, I can't come up with a good reason why

we use /30s either.

My questions:

1. At the Access Layer, what are the benefits of using /30s, over 
subnets with shorter prefixes.

2. Are there administrative benefits to such a policy?

3. Are there routing or switching benefits to such a policy?

4. Is it a best practice policy?

Thank you for comments.

Steve Lim - Network Engineer (Michigan)
Corecomm -An ATX Communications Company
Support Bacteria, it's the only culture
everyone has in common. -limmer

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