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The four layers of the TCP/IP protocol suite

The following diagram serves to illustrate the four layers of the TCP/IP protocol suite and to build upon the knowledge we gained during our study of the OSI reference model to illustrate data flows through each of the layers during the process of encapsulation. At each layer, data is passed down to the layer directly underneath and becomes an SDU or payload at that lower layer. A header, containing information that this lower layer requires, is then added to the SDU/payload, before the process is again repeated for the layer below it. Once the data reaches the Link Layer, it is transmitted across physical media before the reverse process of decapsulation begins:

Let's briefly discuss each of the layers that comprise the TCP/IP protocol suite:

Application Layer: At the very top of the protocol stack exists the Application Layer. The programs that we use every day on our desktop computers and mobile devices exist at this layer. For example, every time you request a web page in your browser, you use the Hypertext Transfer Protocol (HTTP) or Hypertext Transfer Protocol Secure (HTTPS) protocol. Protocols at this layer create data that needs to be transmitted to or received from other internet hosts.
Transport Layer: The Transport Layer exists just below the Application Layer, and serves to provide the means for Application Layer protocols above it to transfer data. Devices such as desktop computers and mobile devices also run Transport Layer protocols. There are two well-known protocols at this layer—TCP and the User Datagram Protocol (UDP). TCP provides connection-oriented transmission of data, requiring a connection to be set up between internet hosts before data can be transmitted, but also providing features such as reliable, in-sequence delivery of data. UDP, on the other hand, is a connectionless protocol that does not require any setup before data can be transmitted, but also does not offer features such as guaranteed delivery of data. Applications access the services of Transport Layer protocols (and, by extension, lower layer protocols) through logical ports. For example, the HTTP protocol uses the well-known TCP port 80. The concept of logical ports and which protocols are associated with which well-known ports will be discussed in more detail later.
Internet Layer: The Internet Layer exists just below the Transport Layer, and provides the service of moving data from the Transport Layer across networks, using forms of internet addressing. IP has become the most utilized protocol at this layer, and you are certain to deal with IP addresses from both version 4 of the protocol, IPv4, as well as version 6, IPv6. Other protocols that exist at this layer are the Internet Control Message Protocol (ICMP) and the Internet Group Management Protocol (IGMP). Devices that operate at this layer include routers and layer 3 switches.
Link Layer: At the bottom of the TCP/IP protocol suite, we will find the Link Layer. This layer operates only on the local segment that a host is physically connected to, and is responsible for delivering data between devices that are connected in the same local segment/network. Protocols at this layer include the Address Resolution Protocol (ARP), Ethernet, and the Neighbor Discovery Protocol (NDP).

Now that we've covered the services that each layer in the TCP/IP protocol suite provides, let's see how applications can use these layers to actually communicate.