Some protocols and applications (for example DNS or joining multicast
groups via socket.setsockopt) need a binary representation of an IP
address. For IPv4 addresses, socket.inet_aton can be used. On some
platforms supporting IPv6, socket.inet_pton can be used for IPv4 and IPv6
addresses. However, this excludes platforms with  (not socket.has_ipv6 or
sys.platform == 'win32').
A property ("packed") should be defined for the IPv4 and IPv6 classes and
return a binary representation (bytes in Python3+, str otherwise).
Additionally, as soon as the code is ported to python3, calling
IP(bytes_instance) should yield an IP i so that i.packed == bytes_instance .