Which application operates on the well-known port number 53?

The application that operates on well-known port number 53 is the Domain Name System (DNS). DNS is critical for the functionality of the internet as it translates human-readable domain names (like www.example.com) into numerical IP addresses that computers use to identify each other on the network. This port is crucial for conducting both DNS queries and responses.

Port 53 is reserved specifically for DNS services, and most DNS servers will use this port to listen for requests. When an application needs to resolve a domain name, it sends a query to a DNS server via this port, allowing it to retrieve the correct IP address.

Other applications operate on different port numbers: FTP typically uses ports 20 and 21 for file transfer services, HTTP operates on port 80 for transmitting web pages, and SMTP uses port 25 to send and receive emails. Thus, these applications are not associated with port 53, reinforcing why DNS is the correct answer.