The Internet

The Internet was created by the Advanced Research Project Agency (ARPA) and the U.S. Department of Defence for scientific and military communications, and was called ARPANET. In 1969 it consisted of four network nodes, connected by a phone line.

The Internet is a "network of networks", and is made up of millions of computers, and other hardware, such as smartphones, PDAs, TVs, Blu Ray players, gaming consoles, printers and light bulbs.

The physical structure of the Internet uses fiberoptic cables, satellites, phone lines and other telecommunications media.

TCP/IP

Every computer and network on the Internet uses the same protocols, the Transmission Control Protocol/Internet Protocol, or TCP/IP. No matter what type of computer system you connect to the Internet, if it uses TCP/IP, it can exchange data with any other type of computer. TCP/IP was developed to tolerate unreliable sub-networks and the protocol guarantees proper transmission of data, since the physical network can't.

There are four TCP/IP layers:

• The application layer provides applications the ability to access the service of the other layers and defines the protocols that applications use to exchange data.

• The transport layer is responsible for making sure that complete messages are delivered end to end.

• The network/Internet layer is responsible for routing messages from one place to another. All routers on the Internet run the IP protocol.

• The bottom layer is the Physical Layer. This is responsible for actually translating the software message into a physical representation and putting them on the wire (or through the air in a wireless network or fiberoptic wire).

There are many different protocols at each lever, here are some representative protocols for the Internet:

Layer	Protocol
Application layer	HTTP, telnet, ftp, email, voip
Transport layer	TCP, UDP
Network layer	IP
Physical layer	Ethernet, WiFi, ATM, X.25, Frame Relay

Domains and DNS

In addition to an IP address, most Internet hosts, or servers, have a domain name address. A domain name identifies the type of institution that owns the computer. E.g. an Internet server owned by NTNU might have the domain name ntnu.no. The domain name is itself made up of name levels so that .no is Norway, and ntnu is NTNU's web site. Some enterprises have multiple servers, and identify them with subdomains, such as idi.ntnu.no.

Top level domain	Definition	For use by
.com	Commercial	Businesses
.edu	Education	Universities
.gov	Governments	U.S. Federal government agencies
.int	International	Organisations established by international treaties
.no	Norway	Norwegian organisations
.mil	Military	U.S. military
.io	British Indian Ocean Territory	British Indian Ocean Territory organisations and modern web companies
.net	Netwrok	Network providers
.org	Organisations	Non-profit or miscellaneous organisations

As far as the Internet is concerned, the symbolic machine names are "eye candy" for human consumption. IP addresses are necessary for computer communication. The IP addresses are provided by Domain Name Server (DNS) computer's "phone lists" that map symbolic names (ntnu.no) to their IP. DNS machines are responsible for providing the IP mapping and for the upkeep of the database as new machines and IPs are added to the Internet.

DNS exists so that you don't have to remember the IP address, only the domain name (which is easier for us humans to remember). When you write a domain name in your address bar, your computer contacts a DNS server which returns the IP address to your computer. Then the computer accesses the server with that IP address.

Routing

When a computer connects to the Internet, it is connected to a smaller network that is connected to the Internet's backbone, the tier 1 networks. Your request then travels trough, potentially, several networks before reaching its destination. When you use the Internet, your client requests data from a host system. The request and data are broken into packets and travel across multiple networks before being reassembled at their destination.

For robust networks, a transmission protocol must find new routes to a destination as preferred routes don't work very well. This is achieved by dynamic routing, where the routes are selected at the time of transmission, after considering current network conditions. Dynamic routing requires a network architecture devoid of critical sites, whose failure will bring down the entire network. That is, the network cannot be hierarchical.

The Internet was designed on a lattice or graph, where there are a large number of widely distributed paths. The hosts performing routing duties are called routers, of which there are thousands on the Internet.

World Wide Web

Many users confuse the Internet with applications that work over the Internet. There are many such applications, but the five listed below are amongst the most popular:

• Email

• Telnet

• File Transfer Protocol (ftp)

• Internet Relay Chat (IRC)

• The World Wide Web

Timothy Berners-Lee, and other researchers, at CERN nuclear research facility near Geneva, Switzerland laid the foundation for the World Wide Web in 1989. They developed a system of interconnected hypertext documents that allowed their users to easily navigate from one topic to another. Hypertext is a method of organizing information that gives the reader control over the order in which the information is presented.

The key to hypertext is the use of hyperlinks (or links) which are the elements in a hypertext document that allows you to jump from one topic to another. A link may point to another section of the same document, or to another document entirely. A link can open a document on your computer, or through the Internet on a computer anywhere in the world. An entire collection of linked documents is referred to as a web site. The hypertext documents within a web site are known as web pages. Individual pages can contain text, audio, video, and even programs that can be run remotely.