I saw this question on subnettingquestions.com and thought it would be easy!
How many subnets (and hosts per subnet) can you get from the network 10.0.0.0 255.255.240.0?
Unfortunately, the answer they gave is very different to what I got from my calculations.
If someone could answer this (and show their working) I would appreciate it.
Some of the answers I'm seeing below aren't saying exactly what the site says (i.e. the official answer is "4096 subnets and 4094 hosts"). Could anyone elaborate a bit on exactly how you are supposed to reach those values?
For anyone interested in how I tried to calculate it, this is what I did:
255.255.240.0 is a /20 subnet mask, therefore there are 4 borrowed bits (from the 3rd octet).
Number of subnetworks = 2 to the power 4 (as network bits = 4), which gives 16
Number of hosts on each subnetwork = (2 to the power 4) – 2 (i.e. 4 bits in host portion of address), which gives 14.
So I got 16 subnetworks, and 14 hosts on each subnetwork. My answer is out by a factor of 256!
Can anyone see where I'm going wrong? I seem to get the Class C calculations right, but have problems with the others.
With those settings, as @Darius points out, you get a 20bit mask and so:
The maximum number of available hosts is 4094. You cannot, of course, use the first or last IP in a subnet (e.g. 10.0.0.0 and 10.0.15.255) for hosts as these are required for other purposes. The first IP is used for the network address and the last as the broadcast address.
Using exactly your mask, you would strictly speaking only have 1 subnet. Using a mask of 255.255.255.0, you could have 16. You could, of course use other masks to get different numbers of subnets but this gets complex to calculate. This is because it is the mask that creates the subnets.