subnet question

Discussion in 'Server Networking' started by djc, Sep 29, 2004.

  1. djc

    Dodo Guest


    RFCs 760, 791 and 1812 caution against the use of the first and last subnet,
    and in some installations, either the last subnet, or the first and last
    subnet are unavailable. Whether these subnets are usable depends on the
    routing protocols in use on the network and the IP implementation on the
    routing devices on the network.

    In practice an RFC is a guideline--not an officially sanctioned
    standard--and vendors are free to implement workarounds to problems
    highlighted in RFCs. If your network uses UNIX hosts and RIP version 1 as
    its routing protocol, you cannot use the first and last subnet. (In this
    instance, the number of subnets listed in the tables needs to be reduced by
    two in each case.) If your network consists of Cisco routers using OSPF or
    EIGRP, you can use the f irst and last subnet. If you are using Cisco
    routers and IGRP, you can always use the first subnet, but not always the
    last. Of course, there are other combinations that will or will not
    work--either refer to the documentation of your specific devices, or refer
    to the manufacturer to see if the first and last subnet can be used.

    If you are at all unclear on whether your combination will work with the
    first and last subnet, avoid their use. However, if you are short on
    available IP addresses and your configuration supports it, there is no
    reason not to use them.

    Dodo, Oct 1, 2004
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  2. Yea, the material I check at home over the weekend from Cisco was really
    vague. It asked the rhetorical question of wouldn't it be nice to recover
    the first and last subnet, but then when on to only discuss the first subnet
    with no mention of the last at all either pro or con. Anyway, thanks for
    sticking with me through my stubbornness, is always nice in the end to
    have a better understanding of the issue.
    Phillip Windell, Oct 4, 2004
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  3. djc

    djc Guest

    all good info...
    Thanks Dodo and Phillip.
    djc, Oct 4, 2004
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