The Domain Name System (DNS) is the application layer protocol responsible for translating human-readable domain names (e.g., www.cisco.com) into machine-readable IP addresses (e.g., 192.0.2.1). While the question describes the reverse process—resolving IP addresses to domain names—this function, known as a reverse DNS lookup (rDNS), is also a core capability of the DNS protocol. DNS acts as a distributed database, allowing clients to query servers to obtain the necessary addressing information to locate resources on a network.

B. HTTP: This is an application protocol for transferring web content; it relies on DNS to resolve a domain name before it can initiate a connection.

C. UDP: This is a transport layer protocol. DNS queries typically use UDP for transport, but UDP itself does not perform the name resolution.

D. TCP: This is a transport layer protocol. DNS may use TCP for large data transfers (like zone transfers), but it is not the resolution protocol.

1. Cisco Systems, Inc. (2023). IP Addressing: DNS Configuration Guide, Cisco IOS XE Gibraltar 16.11.x. In the "Information About DNS" section, it states, "The Domain Name System (DNS) is a distributed database in which you can map hostnames to IP addresses through the DNS protocol from a DNS server." This document also covers reverse lookup zones.

2. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. In Chapter 2, Section 2.5, "The Domain Name System," the text explicitly identifies DNS as the protocol that provides the directory service of translating hostnames to IP addresses (p. 137).

3. Mockapetris, P. (1987). RFC 1034: Domain Names - Concepts and Facilities. IETF. Section 2.1, "Name space," and Section 2.2, "Domain name space," define the fundamental purpose of the domain system as providing mappings between names and other data, with the primary example being host addresses.