I believe that the conversion to the next generation Internet protocol, IPv6, will eventually happen. Microsoft is even trying to convince NAT router manufacturers to support IPv6, which is fantastic. I've also written about how to enable IPv6 on Mac OS X. In order to prepare for the IPv6-enabled future, applications need to support IPv6. It turns out that this is not as difficult as you might think. HP has an IPv6 Porting Guide that is a very good reference. It is aimed at HP-UX, but most of the information is relevant for all variants of Unix, including Mac OS X and Linux. There is also a HOWTO about porting applications to IPv6 that may be more appropriate if you need a tutorial instead of a reference.