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