Java & Windows (7): Reliably getting IPv4 netmask?
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Thread: Java & Windows (7): Reliably getting IPv4 netmask?

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  1. #1
    Join Date
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    Java & Windows (7): Reliably getting IPv4 netmask?

    I've run into a known bug with Java 6 on Windows. My understanding is that the normal way to get the netmask is to look up the network prefix length and do some bit shifts. The problem is that on Windows the prefix length is often returned incorrectly, so we get a 128 when we should get a 24 or 20.

    At http://stackoverflow.com/questions/1...ask-using-java, it is suggested to put -Djava.net.preferIPv4Stack=true on the Java command line. Unfortunately, on Windows 7, adding that as either a VM parameter or on the Java command line seems to have no effect.

    (a) Does anyone know any OTHER work-arounds for this problem that might still work on Windows 7?

    (b) Alternatively, is there an entirely different way to get the netmask that is reliable?

    Thanks!

  2. #2
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    Re: Java & Windows (7): Reliably getting IPv4 netmask?

    Cross posted here
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  3. #3
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    Re: Java & Windows (7): Reliably getting IPv4 netmask?

    Quote Originally Posted by keang View Post
    Cross posted here
    Yeah, sorry about that. I didn't before know the rule about cross-posting. Here's the complete list:
    http://stackoverflow.com/questions/9...g-ipv4-netmask
    http://www.codeguru.com/forum/showthread.php?t=521196
    https://forums.oracle.com/forums/thr...47643&tstart=0
    http://www.coderanch.com/t/567601/so...ly-getting-IPv

    I think this is a hard question, so I'm trying to increase my odds of getting ANY answer. I'm worried that there may be no solution to this problem, because I'm running into a bug in Java itself.

  4. #4
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    Re: Java & Windows (7): Reliably getting IPv4 netmask?

    Oops. Can't figure out how to edit a post, so here's a correction to one of the URLs:
    http://stackoverflow.com/questions/9...g-ipv4-netmask

  5. #5
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    Re: Java & Windows (7): Reliably getting IPv4 netmask?

    This answer worked. I needed to put this line of code first in my main method:

    System.setProperty("java.net.preferIPv4Stack","true");

    I don't know why putting it on the command line didn't take, but this worked.

  6. #6
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    Re: Java & Windows (7): Reliably getting IPv4 netmask?

    Thanks for posting the answer.
    Posting code? Use code tags like this: [code]...Your code here...[/code]
    Click here for examples of Java Code

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