an ephemeral port
/ eh-FEM-er-ul /
When you call a business, you dial their fixed, published number — but the phone you call from can be any phone; its number is just whatever line you happen to be using. Network ports work the same way. A server listens on a fixed, well-known port that clients know in advance (web on TCP port 443, DNS on 53). The client, though, does not need a famous number for itself — it just needs some unused port so the reply can find its way back. That temporary, throwaway port is an ephemeral port.
When your program opens an outgoing connection without first calling bind to a specific port, the operating system automatically picks a free port from a reserved range and uses it as the source port. The Internet Assigned Numbers Authority suggests the range 49152 to 65535 for this, though many systems use a wider range (Linux often defaults to roughly 32768 to 60999). The port is held only for the life of that connection and then returned to the pool, ready to be reused. Because the four-tuple (source IP, source port, destination IP, destination port) must be unique, the ephemeral source port is what lets one client machine open many simultaneous connections to the same server.
Why it matters: ephemeral ports are the quiet machinery behind everyday multiplexing — opening ten browser tabs to the same site works because each connection gets a different ephemeral source port. They also explain a real-world limit: because there are only about 16000 to 28000 ephemeral ports per source-IP-and-destination pair, a single client hammering one server can run out of them, a condition called source port exhaustion that shows up in busy load testers and proxies. The lingering TIME-WAIT state after a connection closes can keep ports tied up and make this worse.
Your laptop loads a page: the connection is 192.168.1.20:54231 to 93.184.216.34:443. You did not choose 54231 — the OS grabbed it from the ephemeral range. When the tab closes, 54231 goes back to the pool for the next outgoing connection.
Servers use fixed ports; clients borrow temporary ones.
Common confusion: an ephemeral port is normally the client's source port, picked automatically. Servers do the opposite — they bind a fixed, known port so clients can find them. Either side can do either, but this is the usual pattern.