The Internet Printing Protocol — a printer, or the CUPS print server on a Mac or Linux box.
IPP is HTTP with a binary message body: clients POST operations like Print-Job and Get-Printer-Attributes to an ipp:// URL, and RFC 8010 requires printers to support HTTP on the IANA-assigned well-known port 631. CUPS, the print system on macOS and most Linux distributions, listens here for both printing and its web administration interface at http://localhost:631/. Nearly every AirPrint or IPP Everywhere network printer answers on 631 as well, which makes it one of the most common open ports on an office LAN.
Expose with care
Fine on a trusted LAN, where it is designed to live. Exposed publicly it leaks the print queue and device details and hands strangers the CUPS admin interface and a print-job submission path — bind CUPS to localhost or the local subnet.
$ curl -sI http://TARGET:631/Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.