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Inet4Address and Inet6Address

Java uses two classes, Inet4Address and Inet6Address, in order to distinguish IPv4 addresses from IPv6 addresses:

public final class Inet4Address extends InetAddress
public final class Inet6Address extends InetAddress

Most of the time, you really shouldn’t be concerned with whether an address is an IPv4 or IPv6 address. In the application layer where Java programs reside, you simply don’t need to know this (and even if you do need to know, it’s quicker to check the size of the byte array returned by getAddress() than to use instanceof to test which subclass you have). Inet4Address overrides several of the methods in InetAddress but doesn’t change their behavior in any public way. Inet6Address is similar, but it does add one new method not present in the superclass, isIPv4CompatibleAddress():

public boolean isIPv4CompatibleAddress()

This method returns true if and only if the address is essentially an IPv4 address stuffed into an IPv6 container—which means only the last four bytes are nonzero. That is, the address has the form 0:0:0:0:0:0:0:xxxx. If this is the case, you can pull off the last four bytes from the array returned by getBytes() and use this data to create an Inet4Address instead. However, you rarely need to do this.

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