Unassigned by IANA; like 12345 it is a keyboard-run number, which is the only reason it ever shows up.
IANA has no registration for 13456 — the registry goes from doip-data (13400, ISO 13400 Diagnostics over IP for vehicles) to bprd (13720, Veritas NetBackup) with nothing in between. Nmap lists the port as unknown. Its only claim to appearing at all is that consecutive digits are easy to type, so it turns up as a scratch or test listener in the same way 12345 does. There is no documented product or malware default.
Expose with care
A memorable number usually means a hand-started service rather than a managed one. Find out what it is before leaving it reachable.
$ nmap -Pn -sV -p 13456 TARGETSeen 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.