Marten.Terpstra@ripe.net writes:
Havard.Eidnes@runit.sintef.no writes:
[...]
* I have for a while looked at these host numbers as somewhat inaccurate, * primarily since this does not take subnetting into account. If you subne t * a class C network number (as some people end up doing, as was mentioned b y * Peter Koch, since sometimes people have a small number of hosts at a give n * site), 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 ma ke * 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%.
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.
If you have the need for two large subnets, you have to use a mask of 2:6 (see below). This is bad, we have in some cases seen subnets of size 50-70 and then had to assign one C address for each of them. If you actually use subnetting 3:5, you have 2^3-2=6 subnets with 2^5-2=30 hosts each. You have a maximum of 180 hosts here (router(s) not taken into account), an unsubnetted network would offer 254, so this is about 71 %. Looking at all possible subnet masks, you get: subnet mask | # subnets | # hosts/subnet | total # hosts | usage -------------+------------+----------------+---------------+------ 1:7 | not allowed| | | 2:6 | 2 | 62 | 124 | 49 % 3:5 | 6 | 30 | 180 | 71 % 4:4 | 14 | 14 | 196 | 77 % 5:3 | 30 | 6 | 180 | 71 % 6:2 | 62 | 2 | 124 | 49 % 7:1 | not allowed| | | Percentage is nothing to worry about too much. More address space would be wasted if you would assign a full class C net for every subnet the requestor operates.
* 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 ;-)
Agreed. Peter