You can recognise a Class B address because the first octet is a number from 128 to 191 inclusive. So any of the below IP addresses are class B.
Network devices recognise a Class B address as such because the numbers are checked in binary and the first number in the first octet always begins with the binary number 10. Check the binary section for more info on that.
Class B addresses lock the first two octets for network numbers. The remaining three octets can be used for host numbers. It works like this:
In the above example 129.23 is the network number and 2.4 is a host on that network.
Please note that Class A, B and C addresses are only called such for historical purposes now and you may find that your company could be allocated a Class A address or even one for your home IP address for your broadband connection. VLSM means that we don’t have to concern ourselves with Classes of address anymore.