*
 * <p>"IPv4 mapped" addresses were originally a representation of IPv4 addresses for use on an IPv6
 * socket that could receive both IPv4 and IPv6 connections (by disabling the {@code IPV6_V6ONLY}
 * socket option on an IPv6 socket). Yes, it's confusing. Nevertheless, these "mapped" addresses
 * were never supposed to be seen on the wire. That assumption was dropped, some say mistakenly, in

   * <ol>
   *   <li>Use sendfile(2) or equivalent. Requires that both the input channel and the output
   *       channel have their own file descriptors. Generally this only happens when both channels
   *       are files or sockets. This performs zero copies - the bytes never enter userspace.
   *   <li>Use mmap(2) or equivalent. Requires that either the input channel or the output channel