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Variable Length Subnetting Example

For example, if you receive the Class B network ID, 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 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 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.

This page clarifies issues surrounding subnetting IP networks by providing a standard subnet table. This table includes subnetting for Class A, B, and C networks, as well as Network IDs, host ranges and IP broadcast addresses with emphasis on Class C subnets.
This memo is intended as an informational companion to Subneting RFC [1] and the Hosts Requirements RFC [2].

Further Information

The growth of networking since the time of STD 5, RFC 950 and STD 3, RFC 1123 has resulted in larger and more complex network subnetting. The previously mentioned RFCs comprise the available guidelines for creating subnetted networks, however they have occassionaly been misinterpreted leading to confusion regarding proper subnetting.

This document itemizes the potential values for IPv4 subnets. Additional information is provided for Hex and Decmial values, classfull equivalants, and number of addresses available within the indicated block.