What is the most common protocol used in the network layer?

The most common protocol used in the network layer is the Internet Protocol (IP). The network layer is responsible for the routing of data from the source to the destination across multiple networks, and IP is the principal protocol that accomplishes this task. It defines the addressing scheme needed to uniquely identify devices on a network, allows for the fragmentation of packets for efficient transmission, and supports the delivery of those packets even when they must traverse different types of networks.

IP operates above the data link layer (where protocols such as Ethernet work) and below the transport layer (where protocols such as TCP operate). Its foundational role in the Internet and other networks exemplifies its importance in facilitating communication between devices on diverse networks.

Other options, while essential in their domains, do not serve as the primary protocol for the functions of the network layer. Ethernet is a protocol used at the data link layer for local area network communications; TCP is a transport layer protocol that ensures reliable communication and error-checking but does not perform routing itself; FTP is an application layer protocol used for transferring files. Thus, IP stands out as the most critical and widely recognized protocol at the network layer.