Section 1.7. Application Layer

At the top of the TCP/IP protocol architecture is the Application Layer. This layer includes all processes that use the Transport Layer protocols to deliver data. There are many applications protocols. Most provide user services, and new services are always being added to this layer.

The most widely known and implemented applications protocols are:


The Network Terminal Protocol, which provides remote login over the network.


The File Transfer Protocol, which is used for interactive file transfer.


The Simple Mail Transfer Protocol, which delivers electronic mail.


The Hypertext Transfer Protocol, which delivers web pages over the network.

While HTTP, FTP, SMTP, and Telnet are the most widely implemented TCP/IP applications, you will work with many others as both a user and a system administrator. Some other commonly used TCP/IP applications are:

Domain Name System (DNS)

Also called name service, this application maps IP addresses to the names assigned to network devices. DNS is discussed in detail in this book.

Open Shortest Path First (OSPF)

Routing is central to the way TCP/IP works. OSPF is used by network devices to exchange routing information. Routing is also a major topic of this book.

Network File System (NFS)

This protocol allows files to be shared by various hosts on the network.

Some protocols, such as Telnet and FTP, can be used only if the user has some knowledge of the network. Other protocols, like OSPF, run without the user even knowing that they exist. As the system administrator, you are aware of all these applications and all the protocols in the other TCP/IP layers. And you're responsible for configuring them!