It was a brand new design so I just used IPv6 from the ground up, there were no adaptation/rewrite involved.

The main cost was the time spent reading the various IPv6 RFCs to make sure I understood the details (it mostly mapped well to my IPv4 knowledge but there are differences). Beyond that it was smooth sailing, although I'm regularly annoyed that common tools are rather clunky when dealing with numerical IPv6 addresses. For instance Firefox requires that I put [] around the IP for it to treat it like an IPv6. SSH on the other hand does not understand the address between brackets. On the other hand SCP does requires the brackets (since it uses a : to delimit the path) which is even more annoying than Firefox because zsh treats [] like a match pattern. It's just small things but it does hurt usability.

Oh and ZeroMQ didn't support IPv6 in their experimental UDP implementation so that was some additional work.

Not op, but when I went through the exercise, the only difference was that I changed the type of socket I was creating, and I used `getaddrinfo()` to get a list of addresses to bind to vs just binding to a single entity.

There really isn't a whole lot of work that needs to be done to support IPv6 in an application, especially new build.