tcp/ip(including ipv4 and ipv6) stack is implemented in tcpip6.prt and tcpip.esk. the tcpip.esk is a configuration file for binding higher level protocol to lower level protocol.

in tcpip.esk, there are both ip and ip6, but the ip6 is bind to ip and resolver.
i don't know what's the function of ip and why ip6 is bind to ip while ip6 and ip is on the same level in stack.
the key questions are:
what's the function of ip6?
what's the function of ip?
what's the relation between ip6 and ip?

please help me!
thank you!