I use Request.UserHostAddress, can only get IPv4 address, how to get IPv6 address of a visitor?
I assume you're talking about HttpRequest.UserHostAddress. It will always return the address that was used to connect to you. If the request was made over IPv6, it will return their IPv6 address; if the request was made over IPv4, it will return their IPv4 address.
You should start by investigating why the request is being made over IPv4. And the folks at http://serverfault.com will probably be of more help on that.
An IPv4 address is translated to an IPv6 address by placing the 4 octets of the IPv4 address into the rightmost 4 octets of the IPv6 address.
After inserting the 4 octets of the IPv4 address into the rightmost 4 octets of the IPv6 address, the remaining left-most octets would be 0 in value.
来源：https://stackoverflow.com/questions/9103977/how-to-get-ipv6-address-in-c