Network Layer
Added 26 Jul 2008
The next layer up from the link layer is called the network layer. The most important protocol at this level is IP, the Internet Protocol. Its job is to send packets or datagrams - a term which basically means “blocks of data” - from one point to another. It uses the link layer protocol to achieve this.
Both the network layer and the link layer are concerned with getting data from point A to point B. However, whilst the network layer works in the world of TCP/ IP, the link layer has to deal with the real world. Everything it does is geared towards the network hardware it uses.
An IP address is a “soft” address. It is a bit like calling your office block “Pan- Galactic House” instead of its real address, 2326 Western Boulevard. The former is no use to the postman who has to deliver the letters, unless he can use it to find out the latter. The link- layer Ethernet protocol needs to know the unique hardware address of the specific network adapter it has to deliver the message to and, in case of an error, the address of the one it came from.
To make this possible, the TCP/ IP protocol suite includes link- layer protocols which convert between IP and hardware addresses. The Address Resolution Protocol (ARP) finds out the physical address corresponding to an IP address. It does this by broadcasting an ARP request on the network. When a host recognises an ARP request containing its own IP address, it sends an ARP reply containing its hardware address. There is also a Reverse ARP (RARP) protocol. This is used by a host to find out its own IP address if it has no way of doing this except via the network.
Internet Protocol
IP is the bedrock protocol of TCP/ IP. Every message and every piece of data sent over any TCP/ IP network is sent as an IP packet.
IP’s job is to enable data to be transmitted across and between networks. Hence the name: inter- net protocol. In a small LAN, it adds little to what could be achieved if the network applications talked directly to Ethernet. If every computer is connected to the same Ethernet cable, every message could be sent directly to the destination computer.
Once you start connecting networks together, however, direct Ethernet communication becomes impractical. At the application level you may address a message to a computer on the far side of the world, but your Ethernet card can’t communicate with the Ethernet card on that computer. Physical Ethernet limitations would prevent it,
for a start. It would, in any case, be undesirable for every computer in the world to be connected to one big network. Every message sent would have to be heard by every computer, which would be bedlam.
Instead, inter- net communications take place using one or more “hops”. Your Ethernet card will communicate with another Ethernet device on the route to the final destination. Routing is the important capability that IP adds to a hardware network protocol. Before we come to it, we will look at some other features of IP.