Re: [SLUG] LAN config

From: Ian C. Blenke (icblenke@nks.net)
Date: Wed May 31 2006 - 16:22:12 EDT


steve szmidt wrote:

>On Wednesday 31 May 2006 09:19, Eben King wrote:
>
>
>>Do the {low,high}est address actually _mean_ something special, or are they
>>just typically used that way? What about using x.x.x.x/32 to refer to one
>>host? Is that a pathological case and therefore requires a special case?
>>
>>
>
>There are really three addresses used. The first one to indicate the network,
>like 192.168.0.0. The second to be the gateway, 192.168.0.1.
>
>The highest is what is known as the broadcast address. 192.168.0.255.
>It's used to make announcements to all hosts on that network.
>
>

If memory serves, a long time ago Microsoft at one point liked to use
the all-zeros host address for broadcasting to a segment.

The "all zeros" (192.168.0.0) address is the "network" address, and the
"all ones" (192.168.0.255) is the "broadcast" address.

RFC 1812 suggests that IP stacks should silently discard such addresses
now. If not silently discarded, such packets MUST be trested as IP
broadcasts.

At one point I _swear_ *.0 was used as a broadcast, and was common on a
number of IP stacks.

>The gateway does not have to be on that address, but is commonly so. You can
>use the 1st for a host, though it's not advisable. Using the highest for a
>host is really not a good idea.
>
>

Meh. It's a convention thing. At NKS, all of our gateways are .254.
Nothing wrong with it, but you should keep it the same throughout your
network if you can help it for convention's sake.

<do not read this if you are easily confused>

In the old days, I would stumble upon crazy IP networks with weird masks
like 255.255.255.1. On it, even numbered hosts were on a different
segment than odd numbered hosts. Thankfully CIDR (/24) notation has
saved us from that potential nightmare.

</do not read this if you are easily confused>

Some things tend change over time. Usually driven by "Industry
Acceptance" and "Best Practices".

Thus is the nature of anything designed by Humans.

- Ian

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