In the search for a network that was more fault tolerant, the early Internet designers researched packet switched networks. The premise for this type of network is that a single message can be broken into multiple message blocks, with each message block containing addressing information to indicate the origination point and final destination. Using this embedded information, these message blocks, called packets, can be sent through the network along various paths, and can be reassembled into the original message when reaching their destination, as illustrated in the figure.
The devices within the network itself are typically unaware of the content of the individual packets. Only visible is the address of the final destination. These addresses are often referred to as IP addresses, represented in a dotted decimal format such as 10.10.10.10. Each packet is sent independently from one location to another. At each location, a routing decision is made as to which path to use to forward the packet towards its final destination. This would be like writing a long message to a friend using ten postcards. Each postcard has the destination address of the recipient. As the postcards are forwarded through the postal system, the destination address is used to determine the next path that postcard should take. Eventually, they will be delivered to the address on the postcards.
If a previously used path is no longer available, the routing function can dynamically choose the next best available path. Because the messages are sent in pieces, rather than as a single complete message, the few packets that may be lost can be retransmitted to the destination along a different path. In many cases, the destination device is unaware that any failure or rerouting occurred. Using our postcard analogy, if one of the postcards is lost along the way, only that postcard needs to be mailed again.
The need for a single, reserved circuit from end-to-end does not exist in a packet switched network. Any piece of a message can be sent through the network using any available path. Additionally, packets containing pieces of messages from different sources can travel the network at the same time. By providing a method to dynamically use redundant paths, without intervention by the user, the Internet has become a fault tolerant method of communication. In our mail analogy, as our postcard travels through the postal system they will share transportation with other postcards, letters and packages. For example, one of the postcards may be placed on an airplane, along with lots of other packages and letters that are being transported toward their final destination.
Although packet-switched connectionless networks are the primary infrastructure for today's Internet, there are some benefits to a connection-oriented system like the circuit-switched telephone system. Because resources at the various switching locations are dedicated to providing a finite number of circuits, the quality and consistency of messages transmitted across a connection-oriented network can be guaranteed. Another benefit is that the provider of the service can charge the users of the network for the period of time that the connection is active. The ability to charge users for active connections through the network is a fundamental premise of the telecommunication service industry.