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Summary Connected: An Internet Encyclopedia

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Up: Programmed Instruction Course
Up: Subnetting and CIDR
Prev: Quiz 3 (cont)
Next: Congratulations


Summary Subnetting, and its more modern cousin CIDR, represent the state of the art in IP addressing design. If you intend to do much work with IP addressing, I suggest you familiarize yourself completely with its structure. Otherwise, it will suffice to keep the following points in mind:

  • Hierarchical addressing.

    Internet IP addresses are assigned according to a hierarchy. Unless you are the Internet Assigned Numbers Authority, you will receive a block (or blocks) of IP addresses from an upstream authority. Usually this is your sole or primary Internet provider.

  • Variable length prefixes.

    IP addresses come in blocks, assigned on bit boundaries, and specified using a binary prefix. The prefix states a count of matching bits (from 0 to 32), and the bit values that must match. The longer the prefix, the more bits are specified, and the fewer IP addresses that actually match the prefix. Conversely, shorter prefixes match more IP addresses. Prefixes are conventionally written using an A.B.C.D/N syntax.

  • Restrictions using older routing protocols.

    Older routing protocols, such as RIP version 1, impose restrictions on IP addressing design due to their inability to fully communicate address prefixes. Use of such routing protocols should therefore be avoided. However, when backwards compatibility must be maintained, the IP addresses must be designed according to the more restrictive principles of classful addressing: fixed length subnet masks, contiguous classful networks, and prefixes no shorter than classful network boundaries.

For further reading on Subnetting and CIDR, I recommend reviewing the following links:

In later sections of the course, we'll see how IP addresses are used by routing protocols to provide global Internet connectivity.

Next: Congratulations

Connected: An Internet Encyclopedia


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