antage is the efficient use of scarce address space as there can be a large number of nodes behind only a few Internet visible IP addresses. Internet routers use the network prefix to route traffic to the correct network. Routers within the subnetted environment use the network prefix plus the subnet number to route traffic to the correct subnet. The network prefix plus the subnet number is called the extended network prefix. Therefore, if an IP address belongs to the class B network but its extended network prefix is 24 bits long, then its subnet mask will be 255.255.255.0 and not 255.255.0.0. The bits in the subnet mask whose value is 1 correspond to the extended network prefix and bits with 0 value correspond to the host number. For example, the IP address 140.1.1.1 traditionally belongs to the class B network, i.e a /16 network with the net mask 255.255.0.0. However, if its first 3 octets represent the extended network prefix and only the last octet represents the host number, then it will be represented as 140.1.1.1/24 and the net mask as 255.255.255.0. Here it would be good to mention that although /<prefix length> notation is compact and easier to understand, many modern routing protocols still expect the subnet to be in its traditional dotted-decimal format. Subnet Planning While designing a network, an IP planner must keep in mind present and future requirements of the maximum number of subnets and the maximum number of hosts in the biggest subnet in an organisation. When a solution comprises several solutions and sites, each solution and/or site should have at least one separate subnet that can be further subnetted as per the solution requirements. If there is a need for six subnets to be made out of a /24 network, then 3 bits will have to be used for the subnet number, giving 2 to the power of 3 i.e. 8 subnets, with two subnets remaining spare. Then 5 bits will be used to represent the host addresses and thus there will be a maximum of 2 to the power of 5 = 32 - 2 = 30 hosts in each of the 3 subnets. C