Network layer

This is third layer of OSI model. The data unit at this layer is known as packet. There is no need of the network layer if the two communicating devices lie on the same network. However, when the two devices are connected on different network, network layer is essential for providing source to destination delivery of packets.

This layer receives data from transport layer. It add header to the data and passes data to the data link layer. At the receiver side, network layer receives data from data link layer. It detaches header from the data and passes data to the transport layer.


  • Logical Addressing :- The data link layer provides physical addressing which is use full for a local network. When the packet is destined for a device outside the network, We require other addressing scheme to identify source and destination. Network layer adds header to the data that includes the logical address of the source and destination. It is a 32 bit address that uniquely identifies the device connected to the network.
  • Routing :- Routing is a process wherein a proper path is defined for reach the destination. Routing can be of two types, Static or dynamic. In static routing, the route to be followed by a packet is already set by the network administrator. In dynamic routing, the route to be followed by a packet is decided at the time of transmission of the packet. The rout can be changed based in the available traffic on the network.
  • Inter-networking :- Inter-networking means connecting two or more computer network together. The internet is the best example of inter-networking. There are different types of network that exist in the real world such as LAN, MAN and WAN. These network are interconnected using various networking devices such as bridges, routers and gateways.

Transport layer

This is the fourth layer of the OSI model. The data at this layer is known as segment. The network layer deals with source-to-destination delivery of individual packets and does not recognize any relationship between them. On the other hand, The transport layer ensures that the entire message reach in order and handles error control and flow control at the source-to-destination level.

Transport layer receives data from the session layer. It divides the data into segments with each segment having its own header and passes these segments to the network layer. At the receiver side, transport layer receives packet from the network layer. It detaches header from them and then reassembles the segments into original message and passes that message to the session layer.


  • Service-Point Addressing :- There may be number of program running at the same time on a computer. For this purpose, source-yo-destination delivery should also include process-to process delivery. Data must be transferred from a specific process on one computer to a specific process on another computer and to accomplish this, port address is requires. Thus, transport layer including service-point address in the header. Using these port addresses transport layer delivers the packets to the correct process.
  • Segmentation And Reassembly : A message is divided into segments with each segment having a sequence number. When the message arrive at the destination, with the help of these sequence number, the transport layer reassembles the message correctly. Sequence number also enable transport layer to determine and replace the segments that were lost in the transmission.
  • Connection Control :- Transport layer provides either connection-oriented or connection less service. In connection-oriented service, transport layer established a connection with the transport layer at the destination before transmission. In connection less service. Transport layer treats each segment as individual packet and transmits it to the transport layer at destination.
  • Error control :- Transport layer also performs error checking. Error control takes place end to end. Transport layer at the sending machine ensures that the entire message is delivered at the receiving machine without an error. Segments are re transmitted to achieve error correction.