Portability (1)

Differences between IPv6 stacks
bind(2) ordering constraints
Packet routing if AF_INET and AF_INET6 sockets are present
Handling of port number conflicts

Supported functionalities
Advanced API?  which revision?
Socket options? (get/setsockopt)
AF_INET socket options on AF_INET6 socket/vice versa?

Try to avoid assuming kernel behavior
Take a safer side
use AF_INET socket option on AF_INET socket, AF_INET6 ones on AF_INET6 socket
Use GNU autoconf
Probe systems supported functionalities at compile time