Hi all,

I have confused myself following a VLSM tutorial and need to get my understanding confirmed, would this be the correct format, or am i away off again

172.31.0.0
VLSM this network address for as many subnetworks as possible, with no more than 14 hosts in each subnetwork. Show the subnet mask used.

As the ip address is class b and contains 16 bits in the network portion, this means that we can have 2^16 = 65536 possible networks, to give each of these networks a maximum of 14 hosts we would use the subnet prefix \28 for each subnet.

Is this correct?

Thanks BB

you don't need to know the 65536 possible networks part (which is actually wrong, it would be 4096 -- I'll explain later), as the question doesn't care about that. All you have to know is that you need up to 14 hosts in each network. Each network has 2^n - 2 hosts where n is the number of bits in the host portion. You subtract two for the broadcast and network addresses. Anyway, n=4 allows for this, so 32 - 4 = 28, and /28 is the right mask. The reason there are 4096 networks, however, is that you are borrowing 12 bits from the host portion of the original address block, and that's the "subnet number" field, so to speak. 2^12 = 4096, so there yougo.