Question:
What do subnet masks mean and how do they work?

Answer:
Subnetworks help improve performance, reduce congestion, enhance securityand allow us to isolate networks. On a wide area network, typically eachlocal area network is a subnet. Sometimes, a LAN can be configured to betwo or more subnets.

On an IP internetwork, an address takes the form of a 32-bit number,for instance: 135.200.45.90

An IP address can be viewed as follows:

If we were referring to a Class B address in the above example, the networkportion of the address would have read: 135.200.

On the other hand, if a Class C was the case, 135.200.45 would haveconstituted the network portion.

A subnetwork mask is also a 32-bit number that indicates the composition ofa IP internetwork; i.e. it reveals how many bits form the network portionand how many bits form the host portion.

Consider the following subnet mask:

FF.FF.FF.0 in hexadecimal, or
255.255.255.0 in decimal.

In binary, thisnumber would read:

11111111.11111111.11111111.00000000

Note that the first 24 bits are all 1s (such a subnet is sometimes referredto as a 24-bit mask). This indicates that the first 24 bits of the IPaddress refer to the network portion, and the trailing eight bits refer to thehost portion. Therefore we can have a maximum number of 254 hosts persubnet; in the above example it would be 135.200.45.1 through 135.200.45.254.

In order to decipher network and host portions of an IP address (we assume aClass B address and subnet mask in this example), the following procedure isused:

A logical AND operation is performed between the IP address and subnet mask.For instance: 135.200.45.254 and 255.255.255.0.

In binary, this would be:

10000111.11001000.00101101.11111110AND11111111.11111111.11111111.00000000resulting in:10000111.11001000.00101101.00000000which is:135.200.45.0

Therefore the subnet number is 45 and the network is 135.200.