IP/Subnet question,PLEASE!

[Alon Brodski]

Hello world!

Awhile ago I was asking some basic info about subnet masking.Now when I know
more,I'd like to ask deeper.
I understand the idea behind the subnets ,but I still don't get the
implementation part.
Let's say we have a Class A IP address:84.94.158.20 and we have 12,506 hosts
on a subnet.So how would we know what subnet mask should be?
IP in bin is:
01010100.01011110.10011110.00010100 (I added dots for convinience only)
2 in a power of 14 is 16,384 and in a power of 13 is 8,192.But the 1st is
too many and the 2nd is too few vs 12506 PC's that are given here.So I
started playing with dec numbers and came to the closest number of 206 as a
3rd dec octet of a subnet mask,ie 255.255.206.0 'cos 255-206=49 and 49
times 256=12544 the highest integer that would give us more than 12506
hosts in the subnet here.
So the first thing is I don't know how then you define the EXACT number of
hosts and 2nd if you translate 255.255.206.0 into bin=
01010100010111101001111000010100
11111111111111111100111000000000
Doesn't look OK...but if I use powers example....2 in a power of 14...then
it would be:
01010100010111101001111000010100
111111111111111111000000.00000000
or 255.255.192.0
I think the 255.255.192.0 is the right answer and not 255.255.206.0 but it
would mean that we can only define a subnet much much biger than actual No.
of hosts....ie.16,384 vs.12,506...

So maybe anyone can explain it to me ?

 

[Doug Sherman [MVP]]

I think you got it. You cannot have a subnet mask of 255.255.206.0. By
definition all the binary digits (reading left to right) in a subnet mask
are consecutive ones until they become all zeros. 206 in binary =
11001110 - because of the intervening zeros, it is not a possible subnet
mask.

You will frequently see exam questions similar to the one you posed. The
implied interpretation should be something like 'what is the largest subnet
mask which will allow you to have at least X hosts.' As you demonstrated,
when you are dealing with powers of 2, you are not going to be able adjust a
subnet mask to exactly match some specific number of hosts. On the other
hand, even exam gurus try to be sort of practical. No one would try to plan
a network to accommodate exactly 12,506 hosts - you always plan for more
than you have.

Doug Sherman
MCSE Win2k/NT4.0, MCSA, MCP+I, MVP