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TCP/IP Guide by Charles M. Kozierok

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IP Variable Length Subnet Masking (VLSM)

The main weakness with conventional subnetting is that the subnet ID represents only one additional hierarchical level in how IP addresses are interpreted and used for routing.

It may seem greedy to look at subnetting and say, "What, only one additional level?" However, in large networks, the need to divide the entire network into only one level of subnetworks doesn't represent the best use of the IP address block.

Furthermore, you have already seen that since the subnet ID is the same length throughout the network, you can have problems if you have subnetworks with very different numbers of hosts on them. The subnet ID must be chosen based on whichever subnet has the greatest number of hosts, even if most ...

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