Re: First draft of the European Template for IP number requests
- Date: Thu, 10 Dec 92 10:05:17 N
Marten.Terpstra@localhost writes:
> Havard.Eidnes@localhost 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
