Network Layer

The NETWORK layer is the traffic policeman of the OSI model

Every packet that the TRANSPORT layer has created needs to be sent to the correct machine across the network. Luckily (by design), every machine has a unique address. And so the network layer adds the correct address to every packet.

It also reads the addresses of incoming packets and if they are destined for this machine then it allows them through. Packets not addressed to this machine are ignored. (This is the basic principle of the network ‘firewall’ that tries to block intruders from entering the local network).

Route planning

Whilst the TRANSPORT layer has set up the information into packets, it is the NETWORK layer that works out the best route for the packets to get to their destination.

Fixing errors.

It can be tough on a packet as it wends it way through the networks! Sometimes they turn up partly corrupted. The NETWORK layer tries to fix them before sending them up to the TRANSPORT layer.

Translating packets

Not all network methods use the same size packet, or even the same method of addressing (i.e. they may use a different ‘protocol’ to the local machine). It is the job of the NETWORK layer to convert these ‘foreign’ packets into the correct form for the layers above.

To summarise its tasks:

	Talks to the TRANSPORT layer above and the DATA LINK layer below.
	Addresses the packets for their destination
	Reads incoming packets and only allows the relevant ones through
	Fixes errors inside individual packets (if it can)
	Converts ‘foreign’ packets into the correct form.
	Works out the best route for the packets to use.