ore Network Planning and Optimisation Autonomous System Autonomous System Figure 4.45 Interior and Exterior Gateway Routing Protocols If a planner sees that the subnet requirement is going to increase in the future, then he/she should use 4 bits for the subnet number instead of 3, giving 16 subnets that can be used in the future. The same philosophy also holds good for a number of hosts. It is better and advisable to have a buffer for the future from the beginning as increasing a subnet size later on in a live network will be very difficult and undesirable. It is to be noted that these days routers and protocols follow classless addressing but all-Os and all-1s subnets can also be used. This means that the router and its protocol are able to differentiate between a 10.10.1.0/24 and a 10.10.1.0/27 subnet. The BGP (Border Gateway Protocol) is one such protocol that has this intelligence while RIP-1 (Routing Information Protocol 1) is unable to do SO. RIP-1 follows classful addressing, where it is forbidden that all bits of a subnet number be zeros or ones. In classless addressing, host addresses are made up of remaining bits after the network prefix. Therefore a /28 network has 4 bits for the host address while a /26 has 6 bits. Now considering a /28 network, the maximum number of host addresses can be 2 to the power of 4, which is 16. Out of these, the first address consisting of all-Os represents the base address of the subnet and the last address consisting of all-1s is the broadcast address. Hence these two cannot be assigned to any host machine. Thus, the total number of usable host addresses be comes 14. In general, any network with a prefix In can have a maximum of 2 to the power of (32 - n) - 2 hosts. 4.10 IP Routing Protocols IP packets move from the source to destination based on 'routes' decided by routing protocols running on devices called 'routers'. The Internet comprises multiple networks worldwide interconnected with routers. A group of networks under the same administrative authority and control is