Network Protocols: Real and Imagined 79 If you look at the Class A subnet mask, you will notice that 255 only appears in the first octet. In binary, 255 would be 11111111, meaning that all eight bits are turned on. These "turned on" bits in the subnet mask actually mask out the first octet of any Class A IP address. This tells a computer that the first octet holds the network information. Notice that all the other octets in the Class A subnet mask are 0--this would be 00000000 in binary --and would not mask out the information in the second, third, and fourth octets. This allows these octets to be used for node addresses. Each octet contains eight bits, so in a Class A network, eight bits are used to determine network information, and the other 24 bits are used for node addresses. This is why so many different possibilities are available for node addresses (again, more than 16 million). In the case of Class B networks, only the third and fourth octets are used for node addresses; this is because the first and second octets are masked out by the subnet mask and provide the network information. With only 16 bits available for node addresses, this means that Class B networks supply fewer node addresses (around 65,000). Using this logic, you can see why Class C networks supply so few IP addresses (254). This is because only the fourth octet is reserved for node addressing, and the rest of the octets are used for network information. The subnet mask masks out all the octets in a Class C address except for the last octet. Tip