How do UDP server and client processes work with port numbers?

The server listens on a well-known or registered port; the client picks a dynamic source port (49,152-65,535) and targets the server's well-known port, reusing the same port pair for the whole transaction. The reply swaps source and destination so it reaches the right client app.

UDP Server and Client Communication:

Server Side:

• Applications assigned well-known or registered port numbers
• Server listens on specific ports
• Examples: DNS on port 53, RADIUS on port 1812

Client Side:

• Dynamically selects a source port (49,152 - 65,535)
• Uses well-known port as destination
• Same port pair used for entire transaction

Example - DNS Query:

Client Request:
  Source Port: 49152 (dynamic)
  Destination Port: 53 (DNS well-known)

Server Response:
  Source Port: 53
  Destination Port: 49152

Port Number Rules:

Key insight: The server's response uses the client's source port as the destination, ensuring the reply reaches the correct application.

Go deeper:

• Port (computer networking) — how the dynamic source port and well-known destination port demultiplex UDP traffic.
• RFC 768 — User Datagram Protocol — the spec defining UDP's source and destination port fields.