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OSPF Network Design Solutions, Second Edition by Thomas M. Thomas

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Case Study: VLSMs

In 1987, RFC 1009 was published with the purpose of specifying how a subnetted network could use more than one subnet mask. As discussed earlier in this chapter, when an IP network is assigned more than one subnet mask, it is considered a network with variable-length subnet masks because the subnet masks (prefixes) have varying lengths.

If you recall, the use of VLSM brings benefits to a network and routing that allow for increased routing optimization in the form of a smaller and more concise routing table, known as route aggregation, as well as more efficient use of an organization’s assigned IP address space.

A /16 network with a /22 extended-network prefix permits 64 subnets (26), each of which supports a maximum of 1022 ...

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