The Emergence of Subnetworks

The Internet's original two-level hierarchy assumed that each site would have only a single network. Therefore, each site would only need a single connection to the Internet. Initially, these were safe assumptions. Over time, however, networked computing matured and expanded. By 1985, it was no longer safe to assume that an organization would only have a single network, nor would be satisfied with a single connection to the Internet.

As sites began to develop multiple networks, it became obvious to the IETF that some mechanism was needed to differentiate between the multiple logical networks that were emerging within sites of the Internet's second tier. Otherwise, there could be no efficient way to route data to specific end systems in sites with multiple networks. This is illustrated in Figure 2-6.

Figure 2-6: The emergence of multiple networks, per site, violated the Internet two-level hierarchy.

One answer was to give each logical network, or subnetwork, its own IP address range. This would work, but would be a tremendously inefficient use of the IP address space. It wouldn't take very long for this approach to threaten to completely consume the remaining, unassigned IP address ranges. More immediate impact would be the expansion of routing tables in the Internet's routers. Each network would require its own routing table entry. Clearly, a better approach needed to be found.

The answer was to organize these logical networks hierarchically and to route between them. Sites with multiple, logical networks should, from the Internet's perspective, be treated as a single network. Therefore, they would share a common IP address range. However, they would need their own, unique range of subnetwork numbers.

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