Which one of the following protocols is not used in internet?

Concept

A protocol is a standardized set of rules that governs how data is formatted, addressed, transmitted and received between devices. The Internet protocol suite (TCP/IP) is a layered family of such protocols; the application layer in particular hosts many of the protocols that everyday Internet services rely on. To answer a "which one is NOT used" question, you check each candidate against this suite: if it is a recognized, standardized member of the TCP/IP stack, it IS used on the Internet.

Application

Test each listed protocol against the TCP/IP suite:

• HTTP — the application-layer protocol that carries web pages and web data; the foundation of the World Wide Web.

• DHCP — the application-layer protocol that automatically assigns IP addresses and network configuration to hosts so they can join an IP network and reach the Internet.

• DNS — both a naming system and a protocol: the DNS protocol (over UDP/TCP port 53) resolves human-readable domain names into IP addresses.

• FTP — the application-layer protocol for transferring files between hosts over a TCP/IP network.

Every one of HTTP, DHCP, DNS and FTP is a standardized, widely deployed member of the Internet protocol suite. There is therefore no listed protocol that is NOT used on the Internet, so the only correct response is "None of these".

Cross-check

Each protocol is defined by a published Internet standard — HTTP (RFC 9110), DHCP (RFC 2131), DNS (RFC 1035) and FTP (RFC 959) — and each has an assigned port in active use. Because all four are confirmed Internet protocols, none of them is the odd one out, confirming "None of these" by elimination.