The first port of the dynamic range — on Windows the first RPC service to ask for a port gets it.
RFC 6335 sets aside 49152-65535 as Dynamic/Private ports that IANA never assigns, and both Windows (Vista/Server 2008 and later) and macOS start their ephemeral allocation exactly here. That makes 49152 the single most commonly open port in the range: it is the first slot handed to an RPC service on a Windows box, and consumer routers frequently land their UPnP IGD HTTP endpoint on it for the same reason. Nmap measures it open on about 0.8% of hosts on TCP and far more on UDP, which is why it appears on almost every Windows scan.
Do not expose
Whatever occupies it — MSRPC or a router's UPnP control endpoint — is internal machinery with no business on a public IP; Microsoft's guidance is to allow the range only inside the network.
$ nmap -sV -Pn -p 49152 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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