Which TCP port is related to the Domain Name System (DNS)?

The Domain Name System (DNS) primarily operates using port 53. DNS is a critical component of internet functionality, as it translates human-readable domain names (like www.example.com) into IP addresses that computers can use to identify each other on the network. When a DNS query is made, it typically utilizes User Datagram Protocol (UDP) on port 53 for the requests, while it may use Transmission Control Protocol (TCP) on the same port for larger queries and zone transfers.

The other options correspond to different services: port 80 is used for HTTP web traffic, port 21 is used for FTP (File Transfer Protocol), and port 443 is designated for HTTPS, which is HTTP over SSL/TLS for secure communications. Each port serves specific purposes within the realm of networking, but for DNS, the appropriate port is exclusively port 53.