The PF router guide is well-written and thorough.
Some Internet services such as Nintendo Switch Online require static port translation to operate correctly. Here is an example of static port NAT from a static local IP address:
pass out quick on $wan proto udp from 192.168.0.220 to any nat-to ($wan) static-port
To match correctly, special-purpose NAT rules should be quick and preceed any general-purpose NAT rules. This particular rule also uses parenthesis for the nat-to destination so the translation is updated whenever the dynamically-assigned WAN IP address changes, as described in https://man.openbsd.org/pf.conf#from
If your ISP already supports IPv6, the lipidity.com unofficial router guide sequel is well-written and thorough.
ISPs generally assign an IPv6 prefix to their customers instead of a single address. NAT is therefore not required, though having a publicly routable IP has security implications which make the use of privacy extensions advisable.
/64 is the standard end-user prefix length for IPv6 prefix delegations, but some ISPs provide shorter prefixes such /48 or /56which allows the local network administrator to subnet the delegated address space into multiple local networks. The final prefix assigned to individual hosts on the network should always be /64, so we can form the subnet ID from the bits that must be added to the ISP-delegated prefix to obtain a prefix length of 64.
For a /56 prefix, we must add 8 bits to get 64, which means there are 2^8 (256) subnet IDs available. For a /48 prefix, we must add 16 bits get a 64 bit prefix, which means there are 2^16 (65,536) subnet IDs available.
The additional bits added to the ISP-delegated prefix are called a Site-Level Aggregation ID or SLA ID in various IPv6 documentation.