In an internet, networks are numbered and, within each network, each network interface is numbered. Hence, it is possible to identify each network station (computer) with at least one number which consists of the network number followed by the interface number. This number is known as the IP (Internet Protocol) number of the station. Stations which have more than one network interface will have more than one IP number and have the potential of forwarding packets of information from one network to another. Messages are sent from one station to another station using message formats consisting of a pair of items; (message-header, message-data).
Each message-header contains, among other items, the IP number of the packet destination as well as the IP number of the packet source. Packets are sent from one station to another along a route which is determined network routers. Each router examines the destination address of each packet it receives and consults a routing table to determine which connected network to which it should be sent.
Occasionally, routers will be so busy they cannot forward a packet to another network, or there will be errors in the routing tables so that no route is correct for a packet. In this case, the packet is simply discarded. This type of delivery of packets is called best effort and is unreliable because packets may be lost under certain circumstances. This is not a problem because network reliability depends on higher level conventions, called network protocols, which attempt to retransmit packets when lost.
IP numbers are used to identify internet hosts and must be entered by users
when accessing internet services such as Telnet, FTP, WEB URL's, etc. Multi-digit
numbers are difficult to enter into user interface programs and recall from
memory. Internet developers devised a Domain Name System (DNS) which
consists of a distributed hierarchical database of names which may be used for
most of the IP numbers on the Internet. Internet hosts are grouped together
into domains within an organization and networks are grouped together by type, educational
(EDU), commercial (COM), etc. Finally, the previously mentioned types of networks
are grouped together by country to form the top level of the DNS database hierarchy.
The DNS system automatically converts host names, which are easier to remember, to
actual IP numbers. An example of such use occurs when you refer to the Apple Computer
WEB site using the URL http://www.apple.com.
The important thing to remember about the way the Internet works is that the familiar model of the telephone system (having a dedicated connection from one point in the world to another) does not apply. Information is sent in small packets and the packets may traverse different routes when sent from one location to another. The internet transmission media are constantly being shared by packets from a variety of sources as they make their way (usually) to their destination. Network protocols use the basic internet transmission mechanism to achieve reliable and secure transport of information.