What was the motivation for adding the IPV6_V6ONLY flag?

Question

In IPv6 networking, the IPV6_V6ONLY flag is used to ensure that a socket will only use IPv6, and in particular that IPv4-to-IPv6 mapping won\'t be used for that socket. On many

Answer 1

For Linux, when writing a service that listens on both IPv4 and IPv6 sockets on the same service port, e.g. port 2001, you MUST call setsockopt(s, SOL_IPV6, IPV6_V6ONLY, &one, sizeof(one)); on the IPv6 socket. If you do not, the bind() operation for the IPv4 socket fails with "Address already in use".