Application:
Defines TCP/IP application protocols and how host programs interface with transport layer services to use the network.
Transport:
Provides communication session management between host computers. Defines the level of service and status of the connection used when transporting data.
Internet:
Packages data into in IP datagrams, which contain source and destination address information that is used to forward the datagrams between hosts and across networks. Performs routing of IP datagrams.
Network interface:
Specifies details of how data is physically sent through the network, including how bits are electrically signaled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fibre, or twisted-pair copper wire.
Computer Hardware 