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The Network Layer - Subnetting - Subnet masks

8 important questions on The Network Layer - Subnetting - Subnet masks

How are subnet IDs calculated?

They are calculated by a subnet mask.

What is a subnet mask?

A subnet mask is a 32-bit address used to distinguish between a network address and a host address in IP address. They are normally written out as four octets in decimal. In binary, it is a string of 1s followed by a number of 0s.

Why does a computer need a subnet mask? Use the example given by Sunny's classroom

On a LAN there are two computers: A and B. There is another computer C, that is not on the LAN. Computer A wants to communicate with computer B and with C somewhere on the internet. How does computer A know that B is on the LAN and C is remote? That is because of the subnet mask.

Let's compare A and B
A's subnet mask (binary): 11111111 11111111 11111111 00000000
A's IP address: 00001010 00000001 10010111 00001010
B's IP address: 00001010 00000001 10010111 00000011
C's IP address: 01000000 11100011 10100000 00010111

Computer A is comparing the IP addresses based on the subnet mask. It uses the AND operator to determine this. As you can see above, the first three octets are the same for A and B which is how the Network ID is identified. This shows that A and B are on the same network (LAN). Computer A now knows that Computer B is a local. The last octet is reserved for the host ID in this case, because A and B are on the same network.

Now, Computer A is going to compare the IP address of C based on the subnet mask. As you can see above by comparing the first three octets, the network ID is different. Computer A now knows the network ID is different. Therfore, subnet mask is used by a computer to determine if another (target) computer is on the same network or a different network.