This layer handles the delivery of messages between different networks.
This layer is unnecessary for communications within the same network.
In this layer the segments from the transport layer are divided into smaller pieces called packets

The network layer receives segments from the transport layer and performs the transmission of the received data segments from one computer to another located in different networks. The data units in the network layers are called packets. This is the layer where routers reside and has the following functions:

  • Logical Addressing (IPv4 & IPv6)
  • Path Determination
  • Routing

Logical Addressing is used by the network layer to assign the sender and the receiver’s ip address to each segment which forms an IP packet. This is so that each IP packet can reach its destination.

Routing, is used by the network layer to move data packets from source to destination. This is based on the logical addressing (IPv4 & IPv6). Suppose a scenario as the image on the right, each computer has a unique address. The network layer of the BBC server has added the sender and receiver’s IP address in the packet. In combination with the network mask it is possible to route the received packet to the right address. In this instance it will move to network 2 and to the computer that requested the source.

Path Determination helps to identify the best path from source to destination. Layer 3 devices use protocols such as OSPF, BGP and IS-IS to determine the best path for the delivery of data.

List of protocols running in the presentation layer:

  • IPsec - Internet Protocol Security
  • IPv4 & IPv6 - Internet Protocol
  • OSPF - Open Shortest Path First
  • RIP - Routing Information Protocol
  • BGP - Border Gateway Protocol
  • IS-IS - Intermediate System to Intermediate System

Exam Question

Explain the difference between a routed protocol and a routing protocol

A routed protocol uses an adressing scheme that enables the identification of the network a host belongs to, in addition to the identification of the correct host of this network. Examples of routed protocols are IP, IPX and ApleTalk.
A routing protocol is only used for routers. It helps routers to build and maintain routing tables.

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