Re: [cobalt-users] IP blocks

["WebFusion System Administrator" <graeme.f@xxxxxxxxxxxxxxx>]

David Lynch wrote:
> A little off subject, but perhaps useful to other newbies....
>
> My provider is telling me that for every IP block
> I purchase (they call a "block" 16 ip addresses)
> I have to keep 2 of them as "non-use" IPs... I asked
> why, and got this response:
>
> "In order to do subneting you need an IP for Broadcast and an IP for
> Internal Networking. Even if you would purchase an entire `class c'
> you would lose 2 ip addresses.  It is my understanding that every
block
> requires this type of procedure. Otherwise, the IP's wont be visable."
>
> This sounds like a steamy pile of BS to me, but I really don't know...
> is there anyone on this list that can enlighten me?  Thanks so much.


It's absolutely correct.

A class C netblock is of the form A.B.C.D where D=0-255 strictly
speaking.

A.B.C.0 -> the network itself
A.B.C.255 -> the broadcast address.

If you were to talk to the broadcast address, every machine with a
TCP/IP stack capable *and configured to do so* will answer. Pick a
subnet on your local LAN and try:

ping a.b.c.255

and see what you get. Note though that this depends on what machines are
out there (not all answer broadcast ping) and also on your local netmask
too :) and if yours is not 255.255.255.0 then you are in a variably
subnetted network.

The broadcast address is used by various things. ARP (address resolution
protocol) and SMB clients (like Windows) spring to mind. When a Windows
machine fires up at boot time, as soon as the IP stack loads it sends
out an ARP request for its own address to the local broadcast address.
This allows it to work out if anyone else already has that address. If
no response comes back from anywhere, it loads up and gets on with doing
whatever you want it to do.
SMB clients use a similar process: upon loading they fire a broadcast
request to the local broadcast address to see if there are any SMB
Browse Masters locally - usually a bunch of replies come back and you
can then start accessing shared files.

In your case, a block of 16 gives you 14 usable addresses - the first is
the network address itself, the last is the subnet broadcast address. So
they're right :)

Hope this helps


Best Wishes,

Graeme Fowler
Systems Administrator
graeme.f@xxxxxxxxxxxxxxx

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