 This e-lecture has introductory character and provides an overview of the Internet, its underlying concepts and its main services including the history of the Internet. In particular, we will talk about the central concepts behind the Internet. We will look at Internet services and identities and we'll talk about the basic functions of the Internet. The Internet is the world's largest computer network. Individual computers are linked up to a host mainframe and hosts are linked to each other through high capacity lines leased from the telephone companies. The Internet has a relatively long history so let us look at it first. The development of the Internet goes back to the 1960s when the Advanced Research Projects Agency or ARPA decided to interconnect their mainframe computers primarily for military purposes during the Cold War. In 1980, the military and the scientific parts of the ARPANET were separated leaving the ARPANET to the global scientific community. In 1989, the European Laboratory for Particle Physics or short CERN located in Geneva, Switzerland, connected to the Internet. This research center provided much of the impetus for the adoption of connectivity and distributed computing standards such as TCP-IP. We will talk about this standard later on. In the same year, Tim Berners-Lee, a graduate in physics who had joined CERN in 1984, wrote the first server and client software for what became to be known the World Wide Web or in short 3Ws or W3. The system was first made available in December 1990 and refined over the period until 1993. At this time, a standard language for hypertext creation on the Internet, the hypertext markup language HTML was established. The Internet resides on three types of machines and the software installed on them. There are servers, machines to be contacted by others. There are clients, machines that connect to servers and there are routers, the so-called pathfinder machines. Let's look at servers first. The central machines on the Internet or more precisely the software installed on them are referred to as servers. The server hardware is not radically different from that of a standard workstation. The software, however, is especially designed to access information of specific types. Depending on the software installed, servers may be subdivided into telnet servers, into FTP servers or into World Wide Web servers. Machines that connect to servers, more precisely the software installed on them, are referred to as clients. Depending on the services used, any standard personal computer with Internet access can be used as a client. The primary purpose of the client's software is to request information from appropriate servers. And then there are routers. A router is a machine dedicated to finding functional, un-congested paths for data packets through the Internet. One reason for the growth of the Internet has to do with this addressing system and its hierarchical organization into networks and subnets. Each computer has a so-called IP address or IP number. The IP number is a 32-digit binary number. Can you read that number? Possibly not. For this reason, this 32-bit value is normally represented by means of dotted octets, so eight digits grouped into one figure. One of these octets or bytes can be translated into the decimal code. In our case, the decimal code would be 131 for the first octet, 18 for the second octet, 23 for the third, and 3 for the fourth. Well, and this decimal representation is normally represented in a dotted format again, and so there is eventually the IP number. Well, and this is how it is interpreted. The largest network is indicated by the first number. The first subnet is indicated by the second number and so on. And the last number, the final number on the right-hand side, indicates the computer number in the last subnet. So if a router has to send a packet to this IP number, all the router has to know is how to send the packet to the network with the number 131. The router at network 131 will know how to get the packet to subnet 18 and so on, and eventually the packet will reach computer number 3 within subnet number 23. The interaction between two machines in the internet involves two central notions. The first notion has to do with a standard according to which data is transmitted, the so-called protocol. And then there is an exact specification of the identity necessary, the identity of each machine within the network. The protocol defines a common set of rules and signals that computers on the network use to communicate. The protocol not only tells the machines how to transfer data, but also ensures that the pieces of data that are sent are well-formed. Two protocols guarantee the successful transmission of data via the internet. The first of these is the so-called internet protocol, or in short IP. The IP specifies the format of packets, also called datagrams, and the addressing scheme. It is a set of instructions which specifies what form the data transferred must have, how connections between machines have to be opened, how data has to be transferred, and how the connection has to be closed again. Furthermore, the IP, the internet protocol, regulates that all data must be sent in packets, where each packet is a chunk of data surrounded by an envelope. The envelope specifies the unique address of the receiving machine. And the second protocol, well, that's the transmission control protocol. The TCP, in short, enables two hosts to establish a connection and exchange streams of data. The transmission control protocol guarantees delivery of data and also guarantees that packets will be delivered in the same order in which they were sent. Different packets belonging to the same body of data may arrive at their destination via totally different routes and at different times. For many kinds of interactions, this is not acceptable. To guarantee the right order of the packets, TCP puts sequence numbers in the packets, and these numbers allow the receiving machine to arrange the packets into their original order. Well, and then there are domain names. Now, for the benefit of human beings, each machine on the internet has not only a unique IP number, whose representation we discussed earlier on, but also a unique name. This name consists of dotted domains, linguistics.online.uni-marburg.de. And they have to be read from right to left, where the rightmost domain is always more general than its neighbor to the left. So the rightmost one here is DE, which stands for Germany. And within Germany, you find an institution called the University of Marburg. Within the University of Marburg, you find a virtual part labeled by the domain part online. And finally, within this virtual part of Marburg University within Germany, there is a linguistics department. Often domain names can be much simpler, like the one over here. Google.ru. Well, and they are self-explanatory. This is of course the Russian version of Google. Well, and how does this all work? When humans send messages to an internet domain, the address has to be translated into an IP number. This mapping is done by a specific software called a resolver. It sends a request to a set of machines called domain name servers. And these servers return a valid IP number to the resolver, allowing the source machine to create envelopes for the internet protocol. That's it. Well, and here are the basic functions of the internet. There are numerous functions, and most of you know these functions. And the most prominent and all-encompassing one is of course the World Wide Web. We will look at it in a second. More specific functions are, of course, email services, electronic mail, that is the transmission of messages over communication networks. You've all written emails, haven't you? So I don't have to explain this, do I? Then there is the file transfer protocol service, the transfer of files between remote machines. Because of some limitations of email, it is often practical to use a specific service to transfer files between remote machines. For example, the collaborative work on a file with one or several colleagues requires reliable file transmission. FTP, the abbreviation for file transfer protocol, is such a service. The telecommunication network, or in short, Telnet, allows clients to access remote machines. Using the Telnet service, one can use any machine that allows remote logins as a workstation. This service is particularly useful for reading one's email on a remote machine. And finally, there is, of course, the World Wide Web. This is a system of internet servers that use hypertext as a way of accessing internet facilities. There are several applications that make it easy to access the World Wide Web. There are web browsers, and there are search engines. Even though the term World Wide Web is often used synonymously with the term internet, not all internet servers are part of the World Wide Web and offer these services. So let's look at browsers first. The term browser is the short form for web browser, a software application used to locate and display web pages. The most popular browsers are displayed here with their logos. You know them all, don't you? Well, write them down. I'll give you the solution right now. Here are the labels. Opera, Safari, Google Chrome, Internet Explorer and Firefox. A search engine is a program that searches documents for specific keywords and returns a list of the documents where the keywords were found. Let's look at an example which you all know. Here is, of course, the most well-known search engine, and we typed in the word Linguistics Marburg, and here are the solutions. And we are quite proud that the Virtual Linguistics Compass is on top of the list and our YouTube channel comes next. Although the term search engine is really a general class of programs, the term is often used specifically to describe systems that enable users to search for documents on the web. Typically, a search engine works by sending out a so-called SPIDER. This is a program that automatically fetches web pages. SPIDERS are used to feed pages to search engines. It is called a SPIDER because it crawls over the web. Another term for these programs is Web Crawler. And then there is a program called INDEXER, which reads these documents and creates an index based on the words contained in each document. Each search engine uses a specific algorithm to create its indices so that, ideally, only meaningful results are returned for each query. Well, so much for the basic functions, identities and concepts behind the Internet. This e-lecture has to be considered not only as an overview but also as a snapshot. Only a bit more than two decades after its birth, the Internet has become not only the central global network but something like the knowledge store of mankind. It is growing fast and has been redefining many traditional approaches and services such as telephone, music, film, television and scientific research and, last but not least, education. This e-lecture would not have been possible without the Internet and its services where YouTube, where we will upload this video, is just one of them.