How do computers communicate on different ISPs

Basics of information and communication on the Internet

1. Brief outline of the history and functioning of the Internet

In 1969 the US Department of Defense constructed a computer network called ARPANET (Advanced Research Projects Agency-NET) that linked four computers. In 1972 it was presented to the public. Many universities and research institutions joined this network in the following years. A second generation of this network software, consisting of a whole family of protocols, was adopted and programmed by 1982. Two of its main elements, the Transmission Control Protocol (TCP) and the Internet Protocol (IP), became one name for the entire family of protocols. Great importance was attached to the fact that TCP / IP is "understood" by different types of computers. All specifications for TCP / IP and the services based on it are described in generally and freely accessible documents, so-called RFCs ("Request for Comment").

TCP / IP spread extremely quickly. Scientific institutions in particular integrated it as an essential part of computer-to-computer communication, although today the network of computers that jointly use TCP / IP as the protocol has been called the Internet. Today, the Internet presents itself as a connection between many sub-networks managed by various organizations and companies, as the "network of networks". All important operating systems (e.g. OS / 2, Windows95, MAC OS, UNIX, LINUX) are today able to understand TCP / IP and to communicate with other computers on which other operating systems are running.

If two computers want to communicate with each other, they must also be able to identify the other. All computers on the Internet must therefore have a unique address. Under TCP / IP, the address consists of a sequence of four numbers between 0 and 255, separated by dots, with which theoretically more than 4 billion computers can be uniquely identified. A typical Internet address looks like this: To make things easier for the user, the individual computers also have names. So-called "Domain Name Servers" (DNS) translate these names into the numerical address, the IP number. So called z. B. "" a computer which, as the ".de" shows at the end, is registered in Germany. The actual domain name is "ekd". In this case, the computer name "mail" refers to the computer on which the e-mail is sent. The IP number of this computer could e.g. Be The user does not have to worry about how the data from his computer find its way to the server. It is completely sufficient for the user to know the address or the domain name of the computer that he wants to address. The routers, which regulate the traffic on the data lines, "know" the way to the addressee of the data packets.

Network addresses and domain names are assigned directly from a central point in the USA. Academic institutions e.g. B. are marked with ".edu" (like "education") in the US. The ending ".com" indicates that this is a commercial facility, which can, however, also be set up in a location other than the USA. Domain names outside of the United States are primarily identified by country. Some exemplary IDs are compiled in the following overview:

.at Austria
.au Australia
.ca Canada
.ch Switzerland
.de Germany
.fr France
.uk UK
.se Sweden
.no Norway .it Italy

In the individual countries there are then further central offices where all domain names and network addresses are assigned and administered for the respective country. In Germany this is the DE-NIC in Frankfurt am Main. All domain names with the ending ".de" are registered here. The Internet only works at all if the computers are clearly addressed by name and assigned numbers.

2. How is the connection to the Internet established?

In order to get access to the Internet as a private person, a commercial or a church institution, one primarily needs an "Internet Service Provider", an institution that provides Internet access. Which type of access and which provider is best for whom depends on a number of factors:

  • whether you want to connect just one computer or an entire computer network with a large number of users to the Internet;
  • whether you just want to communicate and request information, or whether you also want to offer information yourself;
  • which Internet providers operate a nearby access point ("Point of Presence");
  • which services are likely to be used for how long.

With a dial-up line, there is no permanent connection to the Internet provider, but only a temporary one, which is established via telephone and modem or an ISDN card if required. Using special software and a specific PPP (Point to Point Protocol) protocol at both ends of the telephone connection, the data is transmitted over the telephone line. This means that the computer is part of the Internet as long as the telephone connection is active. All available internet services can be used during this online time. If the telephone line is disconnected, the connection to the Internet is also lost. For users who only occasionally need access to the Internet, the dial-up line is the cheapest way to access the Internet: Line costs are only incurred if the line is actually used.

The most convenient way of accessing the Internet for the user is the dedicated line, which is also the most expensive way of establishing an Internet connection. This type of connection is only worthwhile if you want to operate a server yourself that has to be permanently accessible or if you want to connect a large computer network (e.g. a complete network) to the Internet. With this connection, a computer or a computer network is connected to the Internet provider via a router and a dedicated line. Your computers are permanently connected to the network, regardless of whether they are currently being used or not. The users of the local network can access all available internet services at any time. At the same time, all Internet users can also access the information you have provided.

3. Internet usage costs

The costs for Internet access are made up of two components: the costs for the telephone connection, which result from the distance to the provider, and the fee for the Internet provider, which the Internet provider charges for access to the Internet. Some telephone companies (Otelo, Arcor, Viag Intercom) have meanwhile switched to billing for Internet access via a minute fee for telephone and Internet access.

Access to the Internet for a single-user computer is available either via the online services T-Online and America Online (AOL), which enable access to the Internet via their own access software, or Internet service providers who offer a direct connection to the Internet. Billing is done either by the minute or by a monthly flat rate. It is not so easy to say which tariff is the cheapest, because it depends largely on the duration of use and on the telephone charges. Private individuals who use the Internet a lot are usually well served with a flat rate (around 30 DM per month). Anyone who only uses the Internet occasionally (less than 10 hours a month) is also billed according to time, whereby the hour is estimated at around 3-4 DM. The current tariffs, which change rapidly, are best taken from computer magazines that regularly compare prices and services in the online market.

Connecting a network with more than 100 workstation computers to the Internet naturally incurs higher costs. These again depend on telephone costs (dial-up line for smaller networks, dedicated line for larger networks) and the costs of the provider. Some providers charge according to "traffic" (ie according to the transferred data volume), others according to the online time, some according to the usable or used bandwidth on the lines and still others offer flat rates. If you want to connect a complete network to the Internet, you should first determine your own needs as precisely as possible, then obtain offers from various providers, check them carefully and finally conclude a contract that should contain an appropriate period of notice.

Since ISDN enables much higher data transmission rates on the telephone line than an analog line, one would think that this would speed up Internet communication considerably and make online time cheaper overall. However, this only applies to a limited extent on the Internet because worldwide data transmission takes place over a large number of different lines and the data only arrives at the user at the speed that the slowest line provides on this route ("bottleneck syndrome"). Nevertheless, if you already have an ISDN connection, it is advisable to access the Internet via ISDN. Fast analog modems (V.90) are not much inferior to an ISDN connection at the current data transmission rates on the Internet.

4. Internet security

As in all areas of life, the internet is not all about nice people. Some unpleasant contemporaries only make themselves noticeable through rude comments, others aim to cause greater difficulties. They spread computer viruses or try to break into other computer systems. Since your own computer can access computers in other parts of the world via the Internet, you should be particularly careful when downloading software. Some important rules:

  • Use a good password: This is probably the most important precaution that a user can take against break-ins by "hackers". Do not use your first name, username, or any other word on your business card. Since there are programs that use electronic dictionaries to attempt break-ins, you should not use a password that can be found in a dictionary. A good password is as long as possible and combines letters (in lower and upper case), numbers and possibly also special characters (e.g. On3H4zT).

  • Do not keep your password in an easily accessible place: Popular "hiding spots" are under the keyboard, on the desk under the desk pad, on the first page in the organizer, etc.

  • Never pass on your password: Once you have given your password to someone else, you no longer have full control over it. Your friend may mention your password to someone else and it is in the wrong hands.

  • Change password at regular intervals: If your password falls into the wrong hands, the unpopular user can only work with it for a limited time.

  • Investigate Unusual Events: If your computer reacts strangely or if you notice anything unusual, do not ignore it. Investigate the matter, inform the system administrator or the network administrator. Your observations can indicate that the system has been breached.

Security issues are also important when connecting an entire network to the Internet. In this case, your own network must be secured against attacks from outside. Many companies and organizations try to use so-called "firewalls" to protect themselves from attacks on their data. The computer network to be protected is not connected directly to the global Internet, but only via an intermediary computer. This has two network connections, one of which is connected to the internal network and the other to the public Internet. The connection between them is established by a corresponding program. This is configured in such a way that only certain types of connections are permitted. Or: only connections from a specific target address into or out of your own computer network are permitted.

Protecting your own IT system from uninvited guests is only one aspect of network security. Another is the security of the information transmitted. Encryption can solve these problems. Information is encoded according to a specific algorithm before it is sent over the Internet. At the receiving end, the incoming message must first be decrypted before it can be used. This works fine as long as the sender and recipient are clearly defined. For example, the World Wide Web browser "Netscape Communicator" can communicate in encrypted form with the servers developed by the same company. Both sides (and only these sides) know how to encrypt and decrypt the information. All of this works fully automatically, with changing keys between the senders and receivers.

5. The main internet services and how to use them

So far, it has been described what the Internet is, how computers are connected to one another, which security rules must be observed and what costs are incurred. But now the question should be answered, how one can use the internet sensibly, which services can be used in the internet and which possibilities this network of networks offers.

Electronic Mail: Email
Electronic mail (or e-mail for short) is probably the most important network service for communication. The structure of an e-mail corresponds to that of a normal letter: There is a letterhead containing the address of the recipient and the sender, a subject, and the actual message. The addressing is done according to the system "[email protected]", whereby the addresses are not case sensitive. So z. E.g. Uwe Maier, whose address is entered on the computer, can be reached electronically at the address [email protected] (or simply [email protected]). The electronic mail that is sent to such an address is automatically stored in a "mailbox" until the recipient has picked it up.

The real strength of email is communicating with other people on the Internet. In addition to personal letters that are sent directly to individual people, e-mail can be used particularly well for disseminating series information. The information that institutions have previously sent to their recipients by serial fax will increasingly be sent via mailing list in the future, as the documents received by electronic mail can be processed electronically and, on the other hand, the high fax costs (especially in Remote zones or abroad), because the e-mails are always sent to the Internet at the local rate via the nearest dial-in point.

With such electronic newsletters not only can costs be reduced, but recipients can also be targeted. However, these serial emails should only be sent to people who want this information - otherwise it is easy to get complaints and protests. Of course, you can also receive such newsletters yourself and thus keep yourself up to date on the latest developments in areas in which you are interested.

However, e-mail can also be used in organizations and companies themselves as a convenient means of communication in a network. Colleagues can work on the same documents and projects - even over greater distances -, exchange data and inform other employees about special developments. When traveling, employees can fetch their e-mails from their mail storage from any point on the Internet, provided they have local access there.

File transfer (FTP)
FTP (File Transfer Protocol) is available for the transfer of data from Internet computers to a PC or vice versa. With FTP, current software packages can be downloaded from large software archives anywhere in the world, e.g. B. from Microsoft, Novell, IBM, Hewlett Packard, but also from hardware and software manufacturers in other countries, free of charge (or for a fee) directly to your own local computer. This is particularly useful when you need to get certain drivers for a certain hardware or software as quickly as possible. Conversely, you can also offer texts, data, facts, computer games, any software package you want for "download".

FTP is also used to update data on World Wide Web servers. If you have a World Wide Web offer on the Internet yourself, you can make changes to the data yourself via FTP. It does not matter where the computer is actually located: You can change your website from a computer in Berlin to a computer in Munich, provided you have local access in Berlin with a service provider. With a special "Username" (or "Login-ID") and a "Password" you can then change your WWW offers in your directory on the computer at the local rate via FTP and the Internet. The special access code with username and password also ensures that only you can change your data, no one else.

The "Usenet News" or "Newsgroups" are discussion forums that collect contributions from participants on a specific topic and make them available to other interested users in a bundled form. The collection of articles on a specific topic is called a "newsgroup". With programs that are easy to use, you can leaf through articles or contribute yourself. There are currently over 20,000 newsgroups on the Internet. Millions of users worldwide participate in these discussion groups. Hundreds of thousands of new articles are published every day. Here you can z. B. get support for a certain hardware or software from other participants or literally discuss "God and the world".

Internet Relay Chat (IRC)
The IRC is a network of computers that provide hundreds of channels on which Internet users worldwide can communicate with one another "in real time". "Chatting" is particularly popular with young people. Many of the channels have a specific topic as their content. But there are also free channels (e.g. the channel "Germany" - German is spoken here) and private channels for closed user groups. In a channel of the Internet Relay Chat you can "talk" to several participants at the same time. It works as follows: A user writes a sentence, as soon as he presses the return key, this message is written to the screen for all subscribers connected to this channel. The name or pseudonym of the participant is prefixed for identification. In this way, conferences can be held worldwide, or you can just meet for a chat.

World Wide Web (WWW)
The World Wide Web is a global hypertext information system, a global "fabric" of documents which in turn refer to other documents. Information can be linked with each other over the whole world via the unique addressing procedure of the Internet: From a computer in Stuttgart you can just as easily get a reference, a so-called "link", to a computer in Frankfurt or Karlsruhe as to a computer in Sweden USA, South Africa or Israel. This turns the WWW into a global information database in which anyone with Internet access can search for information.

The URL (Uniform Resource Locator) is, so to speak, the Internet address of a specific document that is located in a specific country on a specific computer in a specific directory with a specific file name. The concept of the URL can best be explained using the example "": The first part of this URL, "http:" describes the type of transmission protocol, i.e. HTTP (HyperText Transfer Protocol), the transfer protocol in the World Wide Web. The second part, //, gives the name of the server and the domain name. The protocol part ends with a colon, the server part begins with two slashes.

The third part, /mailing/newsletter.html, describes the path to the relevant file. This part starts with a slash; The individual parts of the path name are also separated by slashes. Our example URL points to the newsletter.html file in the / mailing directory.

6. Where can you find what on the World Wide Web?

Information on the World Wide Web can be found either by knowing the URLs of the relevant information from newspapers or magazines, from friends or other sources, or by using one of the large "search engines" (large databases in which you can register WWW information yourself can retrieve). These databases collect WWW addresses, which can either be sorted thematically or accessed using a special database query with a keyword search. The best known thematic search engine, with many different categories, is "Yahoo" ( or, in Germany also "". The best known search engines for keyword searches are "Altavista" (, "Lycos" ( or The data research is - as everywhere on the Internet - free of charge, but you have to put up with advertising on the pages. In the field provided, simply enter any keyword and the server will then give you a list of documents found for the search term, which you can then click directly with the mouse and then be connected to the corresponding server.

7. Offer information on the World Wide Web

As an Internet user, you can not only communicate and request information over the network, you can also provide information to other users. If you want to offer information on the World Wide Web, you can either install your own computer with the appropriate WWW software and connect it to the Internet via a dedicated line via a service provider, or you can install a server (also with its own domain) on another computer that is already permanently connected to the Internet. In addition to your own hardware and software costs, this also saves you the expensive connection via a dedicated line. You can then update the data on this server via a dial-up line and FTP.

In order to be able to present information on the World Wide Web, the documents must be prepared in a certain language, the HTML language. HTML documents essentially consist of an ASCII text that is prepared with certain commands and formatting features, so-called "tags" (markings). The actual display of texts, images and sound documents is done by the browser (Netscape Communicator, Microsoft Internet Explorer), which is available (free of charge) for all system platforms. This makes it possible to prepare information in such a way that this data can be accessed from a wide variety of computers via the respective client software. Since the HTML language is an integrative computer language, in addition to texts and images (including audio files), other Internet services can be integrated: For example, e-mails can be sent from WWW pages and forms can be filled out, which are then either saved in a Database or forwarded by email. A reference to other text formats (Winword, RTF), to newsgroups, images, Excel charts, etc. is also possible.

Since access to a specific HTML document can be precisely recorded on a WWW server via a "log file" (a protocol file), this is an excellent means of finding out who, when and how often the Has read or viewed information and from where the site was accessed. Since only the computer addresses are stored in these log files, no personal data can be collected. These can, however, be collected using the forms.