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  1. #1
    Join Date
    Jun 2004
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    139

    Unusable IP Address

    Why would an Ip address be unusable?

  2. #2
    Join Date
    Apr 2005
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    In a given subnet (let's say a /29 for example), you have 29 network bits, 3 host bits. Those 3 host bits (one, two, and four, along with all-zeros) added together give 8 possibilities in binary. So a /29, you have 8 total IPs. The bottom IP (network address) and top IP (broadcast address) are technically "illegal" to use per established IP standards (IETF RFCs).

    There are some OS's (like Linux) which allow you to use those IPs, in spite of what I said above. An example of an OS that does not allow you to use the top/bottom IPs would be all versions of Windows. You can still use them in Windows only if you change the subnet mask to a bigger mask than your actual allocation. Another example is cPanel; network and broadcast IPs are illegal according to their scripts.

    Hope that helps.

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  3. #3
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    * Moved to Technical and Security Issues...

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  4. #4
    Join Date
    Oct 2006
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    44
    There are also ranges of ip addresses which are reserved of private use. 10.0.0.0/8 is the reserved A class address range, 192.168.0.0 through 192.168.31.255 are private C class networks, and I can never remember the reserved B class addresses. You can use these addresses in a private network, but they won't be accepted on the internet, you'll have to do some type of network address translation (NAT).
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  5. #5
    Join Date
    Jan 2004
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    Quote Originally Posted by spryandrew
    There are also ranges of ip addresses which are reserved of private use. 10.0.0.0/8 is the reserved A class address range, 192.168.0.0 through 192.168.31.255 are private C class networks, and I can never remember the reserved B class addresses. You can use these addresses in a private network, but they won't be accepted on the internet, you'll have to do some type of network address translation (NAT).
    172.16.0.0/12

    I think the OP is more referring to what Tical said however regarding network / broadcast addresses.
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  6. #6
    Join Date
    Jun 2004
    Posts
    139

    Question

    Quote Originally Posted by tical
    In a given subnet (let's say a /29 for example), you have 29 network bits, 3 host bits. Those 3 host bits (one, two, and four, along with all-zeros) added together give 8 possibilities in binary. So a /29, you have 8 total IPs. The bottom IP (network address) and top IP (broadcast address) are technically "illegal" to use per established IP standards (IETF RFCs).

    There are some OS's (like Linux) which allow you to use those IPs, in spite of what I said above. An example of an OS that does not allow you to use the top/bottom IPs would be all versions of Windows. You can still use them in Windows only if you change the subnet mask to a bigger mask than your actual allocation. Another example is cPanel; network and broadcast IPs are illegal according to their scripts.

    Hope that helps.

    Thanks for the eduation. The reason I ask is because I ordered a dedi box that came with 1 IP. I can purchase an additional 4 for $4 months, but now my host is telling me that I would only get a total of 4.

    So I am wondering why one might be unusable now

  7. #7
    Join Date
    Apr 2003
    Location
    NC
    Posts
    3,093
    Quote Originally Posted by Bruin03
    Thanks for the eduation. The reason I ask is because I ordered a dedi box that came with 1 IP. I can purchase an additional 4 for $4 months, but now my host is telling me that I would only get a total of 4.

    So I am wondering why one might be unusable now
    That sounds like a confusion on what they are offering vs what you thought you were going to get.
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