Casey,
The problem is I come from Enterprise networks where I could waste entire
networks... it was definitely a lot easier to summarize then. Now I *have*
to use public IPs and every site is running multi-homed BGP with upstream
and downstream neighbors.
Let's look at this example:
/23 - 24.0.0.0
/26 - 24.0.0.0 Seattle (24.0.0.1-24.0.0.63)
/30 - 24.0.0.64 Wan Link (2 addresses)
Let's say I want to have a link to San Francisco using the /30. For San
Francisco I need about 20 addresses so I'll use a /27, giving me 30
addresses.
The subnets I can use for a /27 end in 0, 32, 64, 96, 128, etc. etc. let's
say I want to use 24.0.0.96. That leaves a lot of room for other addresses.
If I put more /30 links to other networks in that space, it makes it very
hard to summarize. I have yet to see any real world examples of how this is
being done by larger service providers. I think small ISPs get away with
inefficiencies because they don't have that many routes, and larger ISPs
have the advantage of having larger blocks to work with and fast core
routers.
I'm doing some research on this, and I'll see if I can get my upstream
providers to cough up some diags or documentation. If I find anything cool
I'll update the group.
Thanks,
John
----- Original Message -----
From: "Casey Fahey" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>
Sent: Friday, September 22, 2000 9:33 PM
Subject: Re: art of subnetting
>
> Hi John,
>
> With regard to your first point, most WAN links are point-to-point. This
> means that you will not have more than 2 hosts in a subnet. Since a /30
> mask has 4 addresses, 2 of which can be usable for hosts, you want to use
a
> /30 mask on most WAN links.
>
> I am not so sure what you have in mind for your second point... Post a
> clarification to the group if I miss the mark.
>
> You definately want to structure your addressing in such a way that you
can
> summarize as much as possible. The reason for this is to minimize the
size
> of your route tables.
>
> Example : A router will be able to generalize that all addresses in the
> supernet x.0.0.0 will go out interface e0, even if the actual networks out
> there are x.x.0.0, x.y.0.0, and x.z.0.0. The router will just note that
ALL
> traffic in the supernet goes that direction.
>
> FWIW, one last thing to keep in mind is that you will probably be using
> private (10.x.x.x, 172.16.x.x etc.) addresses and then presenting them to
> the Internet using NAT. So you can waste entire subnets if you need to in
> order to help your routers summarize more effectively.
>
> HTH,
>
> Casey
>
> >From: "John Deatherage" <[EMAIL PROTECTED]>
> >Reply-To: "John Deatherage" <[EMAIL PROTECTED]>
> >To: <[EMAIL PROTECTED]>
> >Subject: art of subnetting
> >Date: Fri, 22 Sep 2000 14:01:39 -0700
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> >
> >Here is my dilemna: I could probably answer most questions on CCNP, CCIE
> >written, etc. exams on subnetting, but the most important thing seems to
be
> >real-world experience. I'm mainly looking for more info on the following
> >topics:
> >
> >- /30 for wan links, when/where to use
> >- if you have a /20 (example), where to put your blocks (of course,
> >summarization is a must), and where it's generally better to put /30s
> >according to what two subnets they are connecting
> >
> >All I can really do is look over case studies and real-world examples
that
> >people show me, and learn from my mistakes. I just want to be able to
> >design IP networks and be able to show the results to someone and not
leave
> >any room for improvement ;)
> >
> >John
> >
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