Yah that term isn’t an official term. I just meant it in the sense of a IPv6 prefix. Without knowing more about how your router firewall works / in set up I can’t be too specific.

But in general the way things work with ip addresses is that your ISP provides you with a block of IPv6 address. This block is the prefix/first part of any given ipv6 address on your network. Each host uses that prefix and generates a suffix that it adds in to it in order to generate a full globally reputable IPv6 address.

By default most hosts use the IPv6 privacy extension to random suffixes and cycle through them. This is good for privacy but bad for hosting a public service. You need to turn off the privacy extension and the second half of the IPv6 address will stay static.

Next up you need to write a firewall rule to allow traffic to that globally routable IPv6 address. In an IPv6 system the router does not intercept or rewrite the packets like it does with IPv4. So all a router does is act as a firewall saying “Yup outside hosts can or can’t make inbound connections to certain hosts/ports”

The trick with a consumer IPv6 address space is that just like IPv4 addresses given to your router, the IPv6 prefix can change randomly.

It would be annoying to have to update the firewall rule every time this happened. That’s why the idea of masking matters. You tell the firewall “ignore the prefix of this firewall rule. Just allow or deny based on the static suffix.”

The way to write such rules is different on different firewalls. Most consumer devices don’t have a way to configure such things. Even professional networking equipment mostly makes you use the cli to manage such things.

I hope this helps.