Which port numbers are commonly used for DNS and HTTP services on TCP/UDP?

• A. HTTP uses UDP port 80; DNS uses TCP port 53.
• B. DNS uses UDP port 63; HTTP uses TCP port 80.
• C. DNS uses UDP/TCP port 53; HTTP uses TCP port 80.
• D. DNS uses UDP port 53; HTTP uses UDP port 80.

Answer: (C)

DNS uses port 53 and can operate over both UDP and TCP. UDP handles most ordinary queries because it’s fast and lightweight, but TCP is used when a response is too large for UDP or when reliable delivery is needed, such as during zone transfers between DNS servers. HTTP runs over TCP and uses port 80 by default, since HTTP relies on a reliable, connection-oriented transport. UDP is not used for standard HTTP, and the other common port numbers shown (like 63 for DNS or UDP 80 for HTTP) don’t match how these services are implemented. HTTPS, while not asked about here, uses port 443 over TCP. So DNS with UDP/TCP port 53 and HTTP with TCP port 80 is the correct pairing.