Question : IP Classes

Hi I'm studying for the network + exam, i'm looking at the IP address classes like class A, B, C, D and E.
Well class A has an IP starting from 1-126  with a default subnet mask of 255.0.0.0, and class B is from 128 - 191 with a default subnet mask of 255.255.0.0, and finally class C is from 192 - 223 with a default subnet mask of 255.255.255.0 Well at work our IP is as follows: 172.17.1.2 etc, and the subnet mask is 255.255.255.0 well according to the study material our subnet mask should be 255.255.0.0. and not 255.255.255.0 our network works fine so what could go wrong with the setup we have.

Answer : IP Classes

You can subnet any classful network down to whatever makes sense for your network.
The subnet mask does not always reflect the network class, nor define it. The class is only defined by the highest order bits of the first octect of the network address.
Check out CIDR and Variable Length subnet masking - VLSM. The idea of classful networks is going away.