I was on vacations for a while, I just tested it locally, both tcp and udp, ipv6 server and connecting to 127.0.0.1 resulted in $sock(name).ip being ::ffff:127.0.0.1 from the server's point of view.
For tcp, I'm not the one actually using the value since it's abstracted but it worked perfectly, I'm in charge of using the value for udp and it also worked perfectly without any modification.

Note that one absolutely needs a bind to an ipv6 address for dual socket to work.
If you do not use the -d switch to control the bind yourself on /socklisten and /sockudp, and depending on the mIRC options for binding at alt o > connect > options > port, the bind may not be ipv6 and a simple "/socklisten -u name port" would then not bind to an ipv6 address. This is important and it would be great if it were explained in the help file.

One should always control the bind with the -d switch because iirc, in some situations, the default bind for a socklisten is not according to the binding option. I need to check this more.


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