Host-to-Network (Link) Layer:

The link layer is the lowest layer of the TCP/IP model. The functions of this layer are modulation, line coding and bit synchronization, frame synchronization and error detection. Common protocols include the Address Resolution Protocol (ARP), IEEE 802.3 and IEEE 802.11.

Internet Layer:

The Internet layer is the next layer up from the link layer. Functions include traffic routing, traffic control, fragmentation, and logical addressing. Common protocols include IP, ICMP and IGMP.

Transport Layer:

The Transport layer is the next layer. Functions include message segmentation, acknowledgement, traffic control, session multiplexing, error detection and correction (resends), and message reordering. Common protocols include the Transport Control Protocol (TCP) and User Datagram Protocol (UDP).

Application Layer:

The Application layer is the highest layer in the TCP/IP model and is related to the session, presentation and application layers of the OSI model. The application layer of the TCP/IP model is used to handle all process-to-process communication functions. Common protocols include SSL, FTP, DNS, HTTP, SMTP and many others.