Which port is used by the DNS service by default?

Which port is used by the DNS service by default?

• A. 53
• B. 7
• C. 79
• D. 80

Answer: (A)

DNS operates on port 53 by default. It typically uses UDP for standard queries because most responses are small and can be delivered quickly, making lookups fast. If a response is too large for a single UDP packet or for operations like zone transfers between servers, DNS uses TCP on the same port to ensure reliable delivery. The other ports listed correspond to different services—port 7 is for the Echo service, port 79 for Finger, and port 80 for HTTP—so they aren’t used by DNS by default.

DNS operates on port 53 by default. It typically uses UDP for standard queries because most responses are small and can be delivered quickly, making lookups fast. If a response is too large for a single UDP packet or for operations like zone transfers between servers, DNS uses TCP on the same port to ensure reliable delivery. The other ports listed correspond to different services—port 7 is for the Echo service, port 79 for Finger, and port 80 for HTTP—so they aren’t used by DNS by default.