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Subnetworks

Subnetworks provide a way to organize hosts within a network into logical groups. This means one network is divided into several "sub" networks. Subnet routing allows numerous subnetworks to exist within a network. If used the host bits are divided into two groups: subnetwork and host. Therefore a subnetwork address consists of the following three portions.

Class addresses are divided into a network and host portion only. Because of this factor we must introduce the element of the subnet mask. The subnet mask is always configured with a network IP address. 

subnet

Subnet Masks

Another word for subnet mask is net mask which is a 32-bit number (similar to an IP address). There is a one-to-one correspondence between each of the 32 bits in the Internet address. The simplest type of subnet mask is the default subnet mask. By default, each 8-bit field is turned on (255-all binary ones) or off (0-all binary zeros) depending on the address class (A, B, C). Class A, B, and C are the only addresses requiring subnet masks. Since Class D and E addresses do not have hosts they do not require subnet masks. The subnet mask serves two purposes:

• Distinguish the network and host portions of an IP address

• Specify whether a destination address is local or remote      

and

ANDing 

The network portion of an Internet address can be determined by using the AND operation with the Internet address and subnet mask. The ANDing process will be performed twice: when the computer is initialized with its local IP address and local subnet mask, and when it sends information to a destination address with the destination address and the local subnet mask. To use the ANDing function convert your local IP address and subnet mask into binary form. To calculate the corresponding bits use the following rules:

If you notice we simply take each bit of the IP address and calculates it with the subnet mask. The calculation of 1 and 1is 1 and the calculation of 0 and 1 or 1 and 0 is 0. Look at another example showing our second ANDing result:

Note: If you will notice the ANDing results are the same so that means the data is being sent locally and a router was not used.