"it depends". There used to be NATs that did linear allocation or some other weird schemes (e.g. linear, but matching the lower bit of the WAN port to the LAN p

"it depends". There used to be NATs that did linear allocation or some other weird schemes (e.g. linear, but matching the lower bit of the WAN port to the LAN port, for reasons to do with SIP weirdness). These days, due to the misguided narrative that NATs are security devices, random port selection dominates. Certainly it does in linux (which accounts for most home routers), and all the commercial vendors (Juniper, Cisco, etc.). Some of the latter might let you configure it if you have esoteric needs, I haven't dug into that. Tailscale just assumes that port allocation is random.