No IANA assignment and no documented service, though it is seen slightly more often than its neighbours.
51103 lies in the Dynamic/Private range that RFC 6335 closes to registration. Nmap's service database has no name for it but records a measurably higher open-port frequency than most of the 51000s, which is consistent with a peer-to-peer or client application that reuses a fixed high port rather than with any registered service. Without a name in the primary references, the only honest description is an unassigned port with an application squatting on it.
Expose with care
Ports that recur across unrelated hosts usually belong to consumer software opening its own listener; identify the application before allowing it inbound.
$ nmap -Pn -sV -p 51103 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.
Get Beige Box →also available as JSON · Markdown
Corrections or a missing port? Reply @rimrocksystems.