Re: First draft of the European Template for IP number requests
- Date: Thu, 10 Dec 92 17:32:38 MET
I said:
> * ...If you subnet a class C network number... you *always* waste
> * address space, since subnet 0 and -1 and host 0 and -1 can (normally)
> * not be used. Thus, the best utilization one can make of a subnetted
> * class C network number is around 75% (if I haven't made an error in
> * my calculation). If there is a need for two large subnets, the
> * largest potential utilization immediately drops to around 50%.
and Marten Terpstra replied:
> Not quite sure what you mean here. What do you consider the best utilization
> of a subnetted class C address ? If you split up the C net in 32 hosts parts
> (actually 31), you loose hostnumbers 0,32,64,96,128,160,192,224 and 255
> (which is 9 hostsnumbers out of 255 ~ 3.5%). With two large subnets you loose
> hostnumbers 0,128 and 255 which is around 1%. The only thing is that you will
> have to convince people to pack their network numbers as good as possible.
Ok, lets do the arithmetic for these two cases:
for a subnet mask of 0xffffffc0, you lose
0-63 (subnet zero)
192-255 (subnet "minus one")
and
64, 127, 128, 191 (various broadcast addresses)
which comes out to just under 50% utilization. With cisco routers, if you
know what you are doing (!) you may say "subnet-zero", and use subnet zero
as an ordinary subnet. This is not without danger.
With a subnet mask of 0xfffffff0, you lose
0-15 (subnet zero)
240-256 (subnet "minus one")
and
14*2 host numbers (0 and "minus one" for subnet broadcasts)
which comes out to just under 77% utilization.
> * It is good to see that the number of subnets is asked for.
>
> Exactly, and I think that the mix of number of hosts and subnets is a good
> indication for the registries to base the assigments on. I do not think that
> one should simply give whatever they ask for. We have had more than one case
> where people had 1500 hosts on 50 subnets and asked for 50 class Cs. You
> really want these people to only use up 8 or maybe 16 Cs. Besides if you
> compare the hosts and subnet predictions together with the number of nets
> they request, you get a fair idea whether of not they have any idea what they
> are doing ;-)
Yes, but with other (older) routing protocols than OSPF, this all depends
on the (planned?) topology of the network, since you can't tie together
subnets of a subnetted network with pieces of another network.
- Havard
