Cisco Certified Network Associate (CCNA) Practice Exam

The protocol that operates on UDP port 53 is the Domain Name System (DNS). DNS is essential for translating human-readable domain names, like www.example.com, into their corresponding IP addresses, which are required for locating and accessing resources on the internet. It utilizes UDP for most of its queries due to the nature of the requests, which typically involve small amounts of data and require quick responses.

Using UDP allows for faster communications since it does not establish a connection before data transmission, making it suitable for the rapid query-response nature of DNS. If a query or response is lost, the application can simply resend the request without needing a complicated handshake process.

While some protocols may also use port 53, such as DNS over TCP for larger responses or zone transfers, the primary and most recognized use of UDP port 53 is for DNS service. Other protocols, such as FTP, TFTP, and SNMP, operate on different ports and serve different purposes in the networking environment; for example, FTP uses ports 21 and 20, TFTP uses port 69, and SNMP uses port 161. This context clarifies why DNS is the correct answer for this question.