Port 631: IPP / CUPS
Port 631 is the Internet Printing Protocol, used to send print jobs and by the CUPS printing system on Macs and Linux.
What runs on port 631
IPP runs over HTTP on TCP 631 and is how modern computers talk to printers, including AirPrint from iPhones and Macs. CUPS, the print system in macOS and Linux, serves its own admin page on 631. Some CUPS setups also run cups-browsed on UDP 631 to discover printers.
On a home network
Most network printers accept IPP on 631, and Macs and Linux machines that share printers listen there too. It is normal on the LAN.
If port 631 is reachable from the internet
Printers and print servers should not be reachable from the internet. In 2024 researchers disclosed a chain of flaws in CUPS, starting with cups-browsed on UDP 631, that could let an attacker plant a fake printer and run commands when someone printed, and large numbers of exposed systems were found. Exposed printers are also abused to print unwanted pages.
How to check whether it is open on your network
Confirm no router rule forwards 631 to a printer or computer. On Linux, check whether cups-browsed is running and disable it if you do not need printer discovery.
To find out whether port 631 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.