IP Subnetting: Three-Level Hierarchical IP Subnet Addressing
As I mentioned earlier, subnetting adds an additional level to the hierarchy of structures used in IP addressing. To support this, IP addresses must be broken into three elements instead of two. This is done by leaving the network ID alone and dividing the host ID into a subnet ID and host ID. These subnet ID bits are used to identify each subnet within the network. Hosts are assigned to the subnets in whatever manner makes the most sense for that network.
Interestingly, the earlier analogy to telephone numbers still holds in the world of subnetting and shows how subnetting changes the way IP addresses are interpreted. For example, a phone number like (401) 555-7777 has an area code (401) ...
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