What layer of the TCP/IP model does SMTP belong?

SMTP, or Simple Mail Transfer Protocol, is a protocol used for sending emails across networks. It operates at the Application Layer of the TCP/IP model, which is responsible for enabling communication between applications on different devices.

The Application Layer provides the necessary protocols and services for end-user applications, such as email clients, web browsers, and file transfer programs. SMTP specifically facilitates the sending, receiving, and relaying of email messages, making it essential for email communication over IP networks.

In contrast, the Transport Layer is responsible for providing reliable data transfer and error-checking mechanisms, using protocols like TCP and UDP. The Internet Layer is focused on routing data packets across network boundaries and is where the Internet Protocol (IP) operates. The Network Layer deals with addressing and delivering data packets across the network. Therefore, SMTP's role clearly aligns with the functions of the Application Layer, making it the correct classification.