Port 9100: Raw printing (JetDirect)
Port 9100 is raw printing: anything sent to it is printed as-is.
What runs on port 9100
HP introduced JetDirect printing on TCP 9100, and nearly all network printers now support it. A computer connects and sends the print job data directly, with no authentication. Printer languages like PJL and PostScript sent this way can also change settings and, on some models, read files from the printer.
On a home network
Almost every network printer and many multifunction devices listen on 9100. That is normal on your LAN.
If port 9100 is reachable from the internet
A printer reachable on 9100 from the internet will print whatever strangers send it. In late 2018 a hacker used exposed printers to print messages on tens of thousands of printers worldwide, and researchers have shown that exposed printers can leak stored documents. Printers should never be reachable from outside.
How to check whether it is open on your network
Confirm your router does not forward 9100 and that the printer is not set up with a public IP. RECON's Port Scan will show which LAN devices accept raw printing.
To find out whether port 9100 is open on something in your home, and whether the internet can reach it, the app scans your Wi-Fi from your phone and checks your public address from the outside. It says what it checked and what it could not. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.