Types of Subnetting:

There are two types of subnetting: static and variable length. Variable length is the more flexible of the two. Which type of subnetting is available depends upon the routing protocol being used; native IP routing supports only static subnetting, as does the widely used RIP protocol. However, RIP Version 2 supports variable length subnetting as well. See 3.3.1, “Routing Information Protocol (RIP)” on page 106 for a description of RIP and RIP2. Chapter 3, “Routing Protocols” on page 95 discusses routing protocols in detail.

Static Subnetting:  Static subnetting means that all subnets in the subnetted network use the same subnet mask. This is simple to implement and easy to maintain, but it implies wasted address space for small networks. For example, a network of four hosts that uses a subnet mask of 255.255.255.0 wastes 250 IP  

addresses. It also makes the network more difficult to reorganize with a new subnet mask. All hosts and routers are required to support static subnetting.

Variable Length Subnetting:  When variable length subnetting is used, the  subnets that make up the network can use different subnet masks. A small subnet with only a few hosts needs a subnet mask that accommodates only these few hosts. A subnet with many hosts attached may need a different subnet mask to accommodate the large number of hosts. The possibility to assign subnet masks according to the needs of the individual subnets will help conserve network addresses. Also, a subnet can be split into two parts by adding another bit to the subnet mask. Other subnets in the network are unaffected by the change.

Variable length subnetting allows you to divide the network so that it is possible to define adequate hosts for each subnet by changing the subnet mask for each network. This can be achieved by configuring the routers accordingly. Please note that not every host and router supports variable length subnetting. With static

subnetting each subnet has the same number of hosts. If it is required to have different numbers of hosts for each network, then variable length subnetting should be used. Only networks of the size needed will be allocated and routing problems will be solved by isolating networks with routers that support variable subnetting. A host that does not support this kind of subnetting would have to route to a router that

supports variable subnetting.

Mixing Static and Variable Length Subnetting:  At first sight, it appears that the presence of a host that only supports static subnetting would prevent variable length subnetting from being used anywhere in the network. Fortunately this is not the case. Provided that the routers between subnets with different subnet masks are using variable length subnetting, the routing protocols employed are able to hide the difference between subnet masks from the hosts in a subnet. Hosts can continue to use basic IP routing and offload all of the complexities of the subnetting to dedicated routers.