jdk7u25 release note has the following description.
http://www.oracle.com/technetwork/java/javase/7u25-relnotes-1955741.html#napi-
win
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Change in Networking API Implementation on Windows platforms
The implementation of the networking APIs has been changed on Windows to use
the SO_EXCLUSIVEADDRUSE socket option by default. This change is necessary to

address anomalies that arise when using both IPv4 and IPv6 applications that
require to bind to the same port.

This change may cause issues for applications that rely on the ability to
have multiple processes bound to the same address and port. When such issues
occur, then use sun.net.useExclusiveBind system property as a temporary
workaround to restore legacy behavior.
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However jdk6u51 release note does not have the description. Therefore jdk6
users cannot know the change.