Most isps, you can’t do that anymore as you no longer have a publically reachable IPv4 address. It moved the ‘just configure your router’ part to their equipment, as they now use CGNAT.
CGNAT doesn't have to draw a random ip:port combo out of a hat for every new connection. Nothing is stopping an ISP from implementing it by taking one ip and assigning ports 1-10.000 to customer A, 10.001 - 20.000 to customer B, and so on. Similarly, nothing is stopping an ISP from adding long-lived mappings to an otherwise-random pool which outlive the initial connection.
Some ISPs offer CGNAT traversal by letting you request a fixed ip:port combo via their self-service website. It's pretty much the same as regular NAT traversal, except that you can't freely pick the outside port. And because the number of people who actually care about it is so small: some ISPs even let you request to be exempted from CGNAT altogether! They'll already have a pool of legacy non-CGNAT customers and a pool of new CGNATted ones, so assigning a handful of nerds to the legacy pool to prevent them from complaining isn't a big deal.
> Nothing is stopping an ISP from implementing it by taking one ip and assigning ports 1-10.000 to customer A, 10.001 - 20.000 to customer B, and so on. Similarly, nothing is stopping an ISP from adding long-lived mappings to an otherwise-random pool which outlive the initial connection.
I’m pretty sure that the scarcity of Legacy IP addresses and port numbers (!) is exactly what stops providers from doing that, at least by default. I’ve seen NAT running out of ports way too many times, and shortening of connection tracking lifetime comes with a whole set of hard to spot bugs.
Can we at least stop claiming that IPv6 is "more complicated" than IPv4 when we're making it a requirement for IPv4 ISPs is to create CGNAT management portals.
It’s gotten much worse.