Socketの実装クラス• Windows Vista以降– java.net. DualStackPlainSocketImpl– This class defines the plain SocketImpl that is used on Windows platformsgreater or equal to Windows Vista. These platforms have a dual layer TCP/IPstack and can handle both IPv4 and IPV6 through a single file descriptor.• Windows Vistaより前– java.net.TwoStacksPlainSocketImpl– This class defines the plain SocketImpl that is used for all Windows versionlower than Vista. It adds support for IPv6 on these platforms whereavailable. For backward compatibility Windows platforms that do not haveIPv6 support also use this implementation, and fd1 gets set to null duringsocket creation.– second fd, used for ipv6 on windows only. fd1 is used for listeners and forclient sockets at initialization until the socket is connected. Up to thispoint fd always refers to the ipv4 socket and fd1 to the ipv6 socket. Afterthe socket becomes connected, fd always refers to the connected socket(either v4 or v6) and fd1 is closed. For ServerSockets, fd always refers tothe v4 listener and fd1 the v6 listener.