[ale] IP Calculations

[tewkewl at mindspring.com]

You have no reason to do it unless you are in a fairly large enterprise envirornment.  Just for an example...look at this.

ds3 serving multiple t1 sites

each serial ip (2 per site) would be assigned from a 30bit network.  These networks might start at 10.1.1.0 and could go to 10.1.1.252
now, each one of those /30's are actually in a class c network that started at 10.1.1.0 (broadcast 10.1.1.255) behind each remote router, you could have a class C starting at 10.1.2.0, 10.1.3.0, 10.1.4.0, etc, etc...and could go all the way to 10.1.255.0.

now imagine that scenario with 40 different core routers.  now each of the 40 routers could summarize it's routes out to the other 39 instead of shuffeling an entire routing table around the network. behind each core router would be 10.1, 10.2, 10.3, 10.4, etc, etc

This helps tremendously in a large environment.  you set static defaults on the edge and your ospf/eigrp tables in the core are tiny!  hardly any overhead at all.

my $.02

-Patrick

Matt Shade <mshade at threekay.com> wrote:
> > Hmm? 255.255.255.4 == 11111111.11111111.11111111.00000100b.

Oops, I missed a place  ;(

>
> > 11111111.11111111.11111111.11110010
> > incorrect, since all the numbers to the right of the last contiguous 1
is
> > not a zero
>
> Well, I've actually seen at least one network subnetted
> that way. I just don't know if it was "correct" or
> not, but it -did- work. Very confusing to add new
> hosts, though. I have no idea what the net admin guy
> was thinking.

Maybe you can do that...I was always taught it was wrong. Maybe it's
"technically" possible, but you "shouldn't" do it?

Anyone?

Matt


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