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javawindows-7netmask

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.

In this solution, 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!

P.S. Here is the bug report that pertains to this.

Solution

  • The -Djava.net.preferIPv4Stack=true VM option should work under any OS. Alternatively, it can be put into Java code as System.setProperty("java.net.preferIPv4Stack","true");. Unless, something (library or whatever) is resetting its true state.