Executive's Basic Guide to Intranets

The Executive's Basic Guide to Intranets (Sidebar)

How TCP/IP Works

At the heart of every intranet --what makes all intranet communications possible-- are two Internet protocols for exchanging information --TCP and IP. Together they form what is known as TCP/IP. Althought they are separate protocols, they are tightly woven together to ensure the most efficient method of communication. TCP/IP is used because intranets (and the Internet) are what is known as "packet-switched" networks. In a packet-switched network, information is broken up and sent in many small packets over many different routes at the same time and re-assembled at the receiving end.

TCP performs the work of separating the data into packets and reassembling them on the receiving end, while IP is responsible for making sure that the packets are sent to the right destination either on the intranet, or on to the Internet. Let's take a closer look at how this actually works:

When you send information across an intranet, the data is broken into small packets (less than 1,500 characters each). When TCP breaks the data up into packets, it calculates and adds a checksum to each packet. The checksum is based on the precise amount of data in the packet.

Each packet, along with the checksum, is put into separate IP wrappers or "envelopes." These envelopes contain information that details exactly where on the intranet --or the Internet-- the data is to be sent. All the wrappers for a given piece of data have the same addressing information so that they can all be sent to the same location for reassembly.

The packets are sent independently through a series of switches called routers. Routers examine all IP wrappers and look at their addresses to determine the most efficient path for sending each packet to its final destination, then send them on to the next closest router on the path to its final destination. Since the traffic load on an intranet often changes, the packets may be sent along different routes and may arrive out of order.

If the router sees that the address on the packet is located inside the intranet, the packet may be sent directly to its destination or it may be sent to yet another router. If the address on the packet is located outside the intranet, on the Internet, it will be sent to another router specifically set up to pass packets out onto the Internet.

TCP also includes the ability to check packets and to determine that all the packets are received. As the packets arrive at their destination, TCP calculates a checksum with the checksum that has been sent in the packet. If the checksums don't match, TCP knows that the packet has been corrupted during transmission. It then discards (drops) the packet and asks that the original packet be re-transmitted. When all the non-corrupt packets are received, TCP uses the header information of the packets --which relay the sequence of how to reassemble the packets-- to assemble them into their original unified form.