7 Layer OSI : Network Layer

The network layer ensures the data transfer between two remote computers within a particular Wide Area Network (WAN). The basic unit of transfer is a datagram that is wrapped (encapsulated) in a frame. The datagram is also composed of a header and data field. Trailers are not very common in network protocols.

As shown in the figure above, the datagram header, together with data (network-layer payload), creates the payload or data field of the frame.
There is usually at least one router on WANs between two computers. The connection between two neighboring routers on the link layer is always direct. The router unpacks the datagram from a frame, only to wrap it again into a different frame (or, more generally, in a frame of different link protocol) before sending it to a different line. The network layer does not see the appliances on the physical and link layers (modems, repeaters, switches, etc.).
The network layer does not care about what kind of link protocols are used on route between the source and the destination.

A serial port or an Ethernet card can be used as a network interface. A network interface has a one or more unique address within a particular WAN.