What port and protocol is used by DNS?

The correct answer is UDP/53, as this is the designated port used for Domain Name System (DNS) services. DNS primarily utilizes the User Datagram Protocol (UDP) for standard queries and responses due to its low latency and efficiency in handling small-sized requests, such as retrieving the IP address associated with a domain name.

When a device needs to resolve a domain name to an IP address, it sends a DNS request over UDP to port 53 on the DNS server. If the answer is too large for a single UDP packet (greater than 512 bytes), the response might be sent via TCP on the same port, but this is an exception rather than the norm, indicating that the primary method of communication for DNS is over UDP.

The other options mentioned are associated with different services: UDP/67 is used for the Dynamic Host Configuration Protocol (DHCP), TCP/20 is used for data transfer in FTP (File Transfer Protocol), and TCP/25 is designated for Simple Mail Transfer Protocol (SMTP), which is used for email transmission. Each of these protocols serves distinct purposes in networking, which highlights the specificity of the DNS service to its correct port and protocol.