How to Subnet a Class B Using VLSM
Subnetting is an efficient way to divide the IP addresses in a classful public or private network in order to segregate networks and preserve the limited number of total IP addresses available. Variable Length Subnet Masking (VLSM) is a type of subnetting that is more complicated, taking a classful network of equal subnet sizes and turning it into a group of heterogeneous and arbitrarily-sized classless subnets. Class B private networks are ideal in medium-size organizations, capable of classness subnets with anywhere from tens of thousands to a handful of hosts.
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Instructions
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Determine how many networks you need and how many hosts you need on each network. Class B can accommodate 2^16 or 65,536 hosts in the classful network, so this should not be a problem. Let's say that one of these networks needs only two users for a router-to-router connection, on the same router that has a Class B network for 200 users.
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Note the larger network and the IP address space it occupies. This is done so that the larger networks are created first in order to avoid overlapping IP addresses, which can cause a multitude of networking problems. In this case, the 200-member network occupies "172.16.0.0" to "172.16.0.255."
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Begin the next network at the end of the highest broadcast address. In this case, for simplicity's sake, the beginning of the next network is "172.16.1.0." Since you need only two hosts, you need to borrow 30 subnet bits to segregate the network, as opposed to only 16 bits for a standard Class B address. This gives you a network address of "172.16.1.0" with a subnet of "255.255.255.252" to ensure only two hosts.
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