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5.3 INTERNET
The Internet is a collection of networking technologies, such as Ethernet, SLIP, PPP and others that allows computers to communicate and exchange information. The concept of the Internet began with ARPANET which was funded as a Department of Defense project in 197x. In 198x the Internet was developed, and began to replace the ARPANET. By the late 1980s the Internet was widespread between most universities, colleges, major companies and government agencies around the world. Finally, the Internet hit widespread public usage by the mid 1990s. Today it is the accepted defacto standard network in the world.
Originally the Internet was used to exchange email and files. It was common to anonymously log into a remote computer, with FTP, and upload and download files. In the early 1990s a number of new applications were developed to make interaction with remote computers easier. For example 'archie' made it easy to search for files by names. 'wais' and 'gopher' were early predecessors to 'mosiac' which then lead to 'netscape'. At that time (about 1993) the face of the internet started to change, thanks to the World Wide Web (WWW). Non-professional users of the internet started to arrive through the America On-Line (AOL) service. This also coincided with the first major case of 'spam', where a legal firm mass mailed advertisements for immigration services. Finally, by the mid 1990s microsoft stopped referring to the Internet as a 'fad'. Today, most people and companies vie for an Internet presence.
This section will outline some of the core concepts that are important when designing applications that use the Internet.
5.3.1 Computer Addresses
Most users are familiar with text computer names, such as 'www.gvsu.edu'. But, these names are only for convenience, and the same computer can have multiple names. Consider the example below. The machine 'claymore.engineer.gvsu.edu' can also be called with 'www.eod.gvsu.edu'. In actuality, each computer on the network has a unique four number address. Both of the names below refer to the same computer with the numerical address '148.61.104.215'. The digits of the address can range from 0 to 255.
When a text computer name is supplied it is converted to a numerical address before network access occurs. Consider the case where a computer name is typed into a web browser. The web browser will then call another computer called a Domain Name Server (DNS). The DNS computer has a database of local and remote computers names and numbers. It will convert the computer name to a number, and then return it to the web browser. The web browser then uses the computer number to connect to the named computer.
The number has four parts. The first two digits `148.61' indicate to all of the internet that the computer is at `gvsu.edu' (we actually pay a yearly fee to register this). The third number indicates what LAN the computer is located on (Basically each sub-network has its own number). Finally the last digit is specific to a machine. This addressing method makes it easy to direct network traffic.
There are different classes on networks. The largest is a class A, in which the entire network begins with the same number such as '148.', allowing up the last three numbers to be used for local network addresses. The next smaller network is class B, in which the first two numbers are designated, for example '148.61.'. A class C network specifies the first three digits, such as '148'61.104.', and can have up to 256 addresses. The 'netmask' indicates how many computers can be on a network. A common netmask is '255.255.255.0' which indicates the local network can have up to 256 computers. A netmask of '255.255.255.254' would indicate that there are only two computers on the network.
When a packet of information is sent it passes through many computers between the sender and receiver. Each of the computers is configured to know where the next computer is upstream and downstream. The 'gateway' is the computer on a local area network that passes a packet out to the Internet. For example, if my computer address is 192.168.1.20, the gateway is probably 192.168.1.254. Any packets travelling to/from the Internet will travel through the gateway computer.
The current standard of four number network addresses is called IPV4. This addressing scheme is ultimately limited, and so the address space is being expanded from four numbers to six in the newer IPV6 standard. This standard also introduces some enhancements for security and other applications.
5.3.2 Computer Ports
On the network information is sent in packets. These are addressed to a computer using the IPV4 address, but they also include a port number between 0 and 65535. The port number indicates what service they are trying to access. In general the first 1000 are allocated to well known and agreed upon services, such as email and web serving. Other port numbers in the low thousands are for less common network services, and numbers in the high thousands are used by user programs. An abbreviated list of common network ports is given below. On any computer the may or may not be active.
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