[lxgr]

Of course NAT allows application layer protocols layered on TCP or UDP to pass through without the NAT understanding the application layer – otherwise, NATted networks would be entirely broken.

The end-to-end principle at the IP layer (i.e. having the IP forwarding layer be agnostic to the transport layer protocols above it) is still violated.

[Hikikomori]

You can NAT on IP protocol as well, just not to more than one per external IP.

[brewmarche]

I guess most people mean NAPT/PAT when they say NAT