I understand that the world is moving toward IPv6 because of IPv4 address exhaustion. I also understand that a server can listen for connections on both IPv4 and IPv6.
My question is, with respect to a web service: under what circumstances would a user connect using IPv6? Does it depend on their ISP?
This is a task which normally is usually performed "behind the scenes", nowadays more companies are choosing to point their domains to an IPv6 server, however this is transparent for the end users, they will not be aware if the website is working with IPv6 or IPv4. The main responsibility here is on the back-end side and the system or network administrator needs to keep in mind several factors to start using IPv6 and the advantages that this can represent.
The network capabilities of the current back-bone infrastructure (Severs, Firewalls, Loadbalancers, Routers).
Security - IPv6 can run end-to-end encryption and IPv6 also supports more- secure name resolution.
Scalability -If the company is expecting to have a huge increase of traffic over their web sites in the near future, IPv6 is the option.
Reduce of processing on Network Devices - An advantage that IPv6 has over IPv4 is its large 128-bit address space, which creates sufficient globally- unique addresses and removes the requirement for NAT, also It enables internet service providers to reduce the size of their routing tables by making them more hierarchical.
At the end, sooner or later all we will have to migrate our resources from IPv4 to IPv6, so, If you can start considering to migrate your web sites to IPv6, you will be making sure the continuity of your services in Internet and you will be avoiding further incompatibility problems.