LAYER NUMBER 3 – THE NETWORK LAYER
FINDING A WAY
“Defines how to transport traffic between devices that are not locally attached”
This must be the easiest layer to define. The job of the network layer is to get your data to any device that does not belong to your local network. I’ll say that again. If your destination is not local, it is the job of the network layer to get your data there. So if you are wondering how your data gets from Milan to Sydney, this is where it happens.
In fact some networking programs, such as the Microsoft networking program, that were designed purely for local area networks (LANs) do not have this layer at all. They don’t need to. All they are concerned about is getting traffic from one device of a LAN to another (all probably in the same building). But getting your data from Italy to Australia takes a bit more doing. For data to get to a remote destination two things are needed:
1. A destination address. Think about it – if a vengeful thug wants to find you, all he needs is your address. Once he knows where you are, he will need…
2. A way there (path).
The network layer does both jobs.
At the network layer each segment arriving from the session layer is labelled with a destination address. (A PDU with network layer headers is called a packet or a datagram). The address has to have a network part that is shared by every device in the receiver’s network, and a host part that is unique to the receiving device. This type of addressing is referred to as hierarchical addressing.
Properly addressed packets are then routed across the internet to the destination. The machines that do this routing are, of course, routers. Routers have several interfaces, and a routing table that shows packets which way to get to their destination.
A common analogy is the postal service. When routing a letter to you, the post office staff use your hierarchical address. They start off with the highest level, which might be country. They will send your letter to the correct country where it will be sorted according to city, sent off to the correct city, and so on and so forth until they get to street level where several people will share your street name but only you have your specific door number. In the same way, routing devices in the internet first route according to the destination network (which you will share with several other devices belonging to your network) and then by the part of that address that is unique to the host.
As part of the header, a source address is also included so the receiving device at the other end of the world knows how to address any datagrams making their way back to the sending device.
Click here to continue to the Data Link Layer