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Variable length subnetting example

For example, if you receive the Class B network ID 131.107.0.0, you do not want all the machines on the network to belong to that network ID, since it supports over 65,000 hosts on the same network. You will want to subnet the network using variable length subnet masks so that it can be segmented into more meaningful and manageable "chunks."
If you want to support about 500 hosts on each subnet, you could take 7 bits of the host ID portion of the IP address and apply them to the network ID. This would be represented as 131.107.0.0/23. Each subnet created would be characterized by the /23 at the end of the IP address, denoting how many bits are used to define the network ID portion of the IP address.
If you add new subnets, or remove old ones, the routers and routing protocols must support the use of subnet masks. If you add a new subnet, such as 131.107.6.0/23 to the network, the router adjacent the subnet must be able to automatically communicate the availability of this subnet using routing protocols that support VLSM.