First draft of the European Template for IP number requests
Havard.Eidnes at runit.sintef.no Havard.Eidnes at runit.sintef.no
Thu Dec 10 17:32:38 CET 1992
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
[ lir-wg Archives ]