Through no fault or accomplishment of my own, all mine come up “stealthed”.
No idea how I did that, if *I* even did.
Maybe it’s just my router.
My router is a Frontier Comms DSL modem/wireless combo, on which UPnP has always been disabled, and the only open inbound ports are for ping (which my ISP uses to diagnose problems so I don't mind that).
"Stealth mode" was not enabled, but I've now enabled it. Normally if a router is contacted on a closed port, it responds saying it's closed. Stealth mode changes that response behavior to silence -- it appears as if the router doesn't exist.
FWIW, enabling stealth mode shouldn't make any functional difference other than you might disappoint the random scans done by the hackers and spammers.