[throwawaykf]

There is a school of thought that details of an internal network topology should not leak out [1] (ostensibly because obscuring that has security benefits), and so NATs should be used even if each machine can have a unique IP address. Even though I worked on P2P apps and NATs were the biggest pain point, I do kinda feel the same... Leaking internal details make me squeamish even if I can think of no obvious risks. I think there is a real chance that home routers will also NAT by default even when IPv6 is common.

And you can bet corporate networks will be NATted. Not to mention data centers.

Another concern is Carrier Grade NATs. These are already being deployed within ISP networks to alleviate the IPv4 address shortage. I worry that once this infrastructure is in place, not only will it delay IPv6 adoption, but the adoption will be uneven, as ISPs with CGNs may hold off transitioning much longer. So even if there is a part of the Internet that is all IPv6, the rest of it might still be behind CGNs, and to connect arbitrary peers we'd still have to deal with NATs.

1. http://lwn.net/Articles/452293/

[nly]

> There is a school of thought that details of an internal network topology should not leak out

That's why we have Privacy Extensions