IPv6 is the next generation of Internet protocols, and has been for well over a decade now. Originally designed because it looked like Internet addresses might be running out, IPv6 has turned out to be largely unneeded because less disruptive solutions to the problem have been designed and implemented in Internet routers (in particular NAT and CIDR). IPv6 does offer some small advantages in other areas, and is implemented as part of advanced, university-based efforts like Internet 2.

Right now, however, IPv6 mainly causes security problems. In the Macintosh world, most of these problems have been with Apple's new 802.11n AirPort Extreme base station. An initial, serious problem was fixed fairly quickly by Apple, and now an additional security fix for the base station has also been issued. The fix addresses a more minor problem, where a malicious attacker could carry out a denial of service attack through the base station. The attack is based on a fundamental flaw in a part of IPv6 which is being eliminated due to its potential use in such attacks.

This issue is a great illustration of the fundamental problem with IPv6 right now: despite being over ten years old, it's relatively new and unproven as a reliable, secure protocol. Since, at this time, it really serves no practical purpose for the Rest of Us, it’s important to disable IPv6, not only in the base station, but under Mac OS X as well. As the book says:

1. Currently, IPv6 is used mostly by a few universities and certain government projects, and is probably not something you’ll need to deal with anytime in the near future. You should usually simply disable IPv6 through Network System Preferences, if your firewall doesn’t do 
   

2. this for you.