Subnet Mask – What is Subnet Mask?

• Subnet mask uses the same format and representation technique as IP addresses.

• Subnet mask has binary is in all bits specifying the network and subnet-work fields, and binary Os in all bits specifying the host field.

• A subnet address is created by borrowing the bits from host field.

• Subnet mask bits should come from the high-order (left most) bits of the host field.

• Various types of subnet mask exist for class Band C subnets.

• The default subnet mask for a class B address that has no subnetting is 255.255.0.0, while the subnet mask for a class B address 171.16.0.0 that specifies eight bits of subnetting is 255.255.255.0. The reason for this is that eight bits of subnetting or 2⁸ – 2 (1 for the network address and 1 for the broadcast address) = 254 subnets possible, with 2⁸ – 2 = 254 hosts per subnet.

• The subnet mask for a class C address 192.168.2.0 that specifies five bits of subnetting is 255.255.255.248 with five bits available for subnetting, 2⁵ – 2 = 30 subnets possible, with 2³ – 2 = 6 hosts per subnet.

 How subnet masks are used to determine the network number

• The router performs a set process to determine the network (or more specifically, the subnetwork) address.

• First, the router extracts the IP destination address from the incoming packet and retrieves the internal subnet mask.

• It then performs a logical AND operation to obtain the network number. In logical AND operation, 1 “ANDed” with 1 yields 1 and 1 “ANDed” with 0 yields 0.

• This causes the host portion of the IP destination address to be removed, while the destination network number remains.

• The router then looks up the destination network number and matches it with an outgoing interface.

• Finally, it forwards the frame to the destination IP address.

• Figure shows that when a logical AND of the destination IP address and the subnet mask is performed, the sub-network number remains, which the router uses to forward the packet.