Unassigned by IANA with no documented service — present in default scans on frequency alone.
21571 falls in IANA's unassigned range 21555-21589 and has no registration. Nmap's table names it 'unknown' with a small but nonzero open-frequency, which is the only reason it appears in the default top-1000 TCP scan. Nothing in the public record ties it to a product, protocol, or malware family. Whatever you find open here was put there by someone locally.
Expose with care
No standard service means no default guidance. Identify the listener and inherit that service's exposure rules.
$ nc -vz TARGET 21571Seen 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.
Get Beige Box →also available as JSON · Markdown
Corrections or a missing port? Reply @rimrocksystems.