The application layer is the seventh and highest layer of the OSI model. It is the layer that is closest to the user. The application layer provides services to the user’s applications. It is the layer that allows users to interact with the network. The application layer is responsible for providing a user interface and for supporting services such as email, file transfer, and remote login.

• Network Virtual Terminal: The application layer allows a user to log on to a remote host. This is done by creating a virtual terminal on the remote host.
• File Transfer, Access, and Management (FTAM): The application layer allows a user to access, transfer, and manage files on a remote host.
• Mail Services: The application layer provides the services for email, such as sending and receiving messages.
• Directory Services: The application layer provides access to a distributed database of information about network resources.
• World Wide Web (WWW): The application layer provides access to the World Wide Web. This is done by using the Hypertext Transfer Protocol (HTTP).

• Hypertext Transfer Protocol (HTTP): HTTP is the protocol that is used to transfer web pages. It is a request-response protocol, which means that the client sends a request to the server, and the server sends a response back to the client.
• File Transfer Protocol (FTP): FTP is the protocol that is used to transfer files between computers. It is a connection-oriented protocol, which means that it establishes a connection before transferring the data.
• Simple Mail Transfer Protocol (SMTP): SMTP is the protocol that is used to send email. It is a connection-oriented protocol.
• Domain Name System (DNS): DNS is the protocol that is used to resolve domain names to IP addresses. A domain name is a human-readable name for a website, such as www.google.com. An IP address is a numerical address that is used to identify a computer on the internet.
• Telnet: Telnet is a protocol that is used to log on to a remote host. It is a command-line interface that allows a user to execute commands on the remote host.
• Secure Shell (SSH): SSH is a protocol that is used to securely log on to a remote host. It is similar to Telnet, but it encrypts the data that is sent between the client and the server.

The client-server model is a distributed application structure that partitions tasks or workloads between the providers of a resource or service, called servers, and service requesters, called clients. Often clients and servers communicate over a computer network on separate hardware, but both client and server may reside in the same system.

A server host runs one or more server programs which share their resources with clients. A client does not share any of its resources, but requests a server’s content or service function. Clients therefore initiate communication sessions with servers which await incoming requests.

In a peer-to-peer (P2P) network, each computer can act as both a client and a server. This means that each computer can share its resources with other computers on the network. P2P networks are often used for file sharing and online gaming.