Private address ranges are a human convention and there have been instances in the past of upstream routers passing them on.[1] Relying on other people to do your filtering for you is a bad idea. I'm going to put the rules in my own router, whether those addresses are (potentially) globally routable or are designated as private.

The use of small private pools has even helped attackers who would inject browser scripts probing the well-known prefixes.[2]

[1] https://serverfault.com/questions/374126/private-ip-getting-...

[2] https://www.bleepingcomputer.com/news/security/new-behave-ex...

Exactly! Duplicating my point in a thread below to drive your point home:

NAT was an added layer on top of firewall rules because inbound ports had to be mapped to a particular host and port since the router would not know which host to send to. This created a default opt out experience because for a port on your machine to get accessed, a packet must pass inbound rules and match a port map table entry.

> it’s broke by default, no configuration necessary.

Which is why all sorts of software needs to deal with bullshit like STUN, TURN, etc, to get peer-to-peer connections working. There has to be all sorts of address discovery.

* https://en.wikipedia.org/wiki/NAT_traversal

And even that won't work once you get into CG-NAT with tends to have two layers of NAT.

How much of the centralization of the Internet has occurred because people can't just talk to each other (by simply firewall hole punching via UPnP/PCP)?