internet (part1)

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The Internet is a vast network of many different computers that are able to talk to each other in spite of the fact that they may be separated by large distances, may be very different computers built by different manufacturers, and may be running many different kinds of operating systems. This is similar to people in (say) Israel, Brazil, Japan, the United States, and Norway, each normally speaking a different language, being able to converse almost instantaneously with each other on very broad ranges of topics!

The computers of the Internet exchange packets of information using addresses that have much in common with ordinary mail addresses. These addresses are called Internet Protocol (IP) addresses.

How is such efficient communication possible? Well, there are two big advantages that the computers on the Internet have over our hypothetical natives of different cultures and countries. The first is that the computers of the Internet are in reasonably constant contact with each other over a set of telephone lines, transoceanic cables, satellite links, and so on that allow rapid transfer of electronic information. The second is that the computers of the Internet have agreed to a common set of understandings that allows them to exchange vast amounts of information even though internally they may speak very different computer languages.

These communications are possible because of a set of protocols, which is a fancy word for mutually agreed-upon rules about exactly how information will be transferred between the computers of the Internet. The two most important protocols that allow networks to cooperate with one another and exchange information are called TCP (Transmission Control Protocol) and IP (Internet Protocol). Collectively, these two protocols are often referred to as TCP/IP.

Local networks can be based on a variety of technologies, but as long as the software layers on top of the network hardware implement the TCP/IP protocol operations, a network can exchange intelligible information with the rest of the Internet.

How does information get transferred between computers on the Internet? A pretty good analogy is that of letters and the usual mail system (the one that computer types call "snail mail", because it is so slow compared with electronic mail!). The information is transferred in what are called electronic packets, which are something like letters containing information. If the packets are like letters, they must have an address. Just as normal letters must have an address on the front to make delivery likely, TCP/IP communication depends on addresses being included in each packet. Not surprisingly, these addresses are commonly termed "IP addresses". So, if someone asks what your IP address is, she is asking the electronic equivalent of what your post office address is.

As these packets of information move through the network, electronic devices called routers use these IP addresses to decide whether to keep a packet in a local network or to forward it to a different network. This is a complex task, because there are many networks that either comprise the Internet, or that are attached to it through some gateway.

Thus, routers and gateways in the network serve much the same function that different levels of post offices serve in the usual mail system, with each level of the postal system deciding whether the letter in question should be delivered locally, or forwarded to another part of the postal system.

The address on a normal letter typically has several lines, each of which supplies different levels of information about the exact location corresponding to the address. In a very similar way, addresses for Internet information packets have four fields that contain numbers and are separated by periods. For example,

222.36.28.37

is an IP address. However, people generally can remember names better than numbers, so it is convenient to associate a name with such an IP address. In this case, the corresponding name is

csep10.phys.upi.edu.

The translation between the numbers used by the network, and the name more commonly used by people is done by a computer called a nameserver. The purpose of a nameserver is to look up addresses, so its function may be likened to directory assistance on the telephone system.

If I specify csep10.phys.utk.edu as an IP address to a network, the first thing that it does is call its "directory assistance" - that is, a nameserver - to get the number associated with that name. (Actually, most computers have a small list of address-name mappings stored locally that they first consult, and only go to an external nameserver if they don't find the address there.) The network uses only the number, but by this method the humans and the computers are both kept happy: the computers in the network just have to deal with the numbers, which is what they do best, and the humans using the computers need only remember more symbolic names like

mycomputer.myplace.mynet.mycountry which is what humans do better. Of course, the humans can also supply the IP address directly to the network as a number. Then, the nameserver isn't invoked. In fact, sometimes the nameservers, since they are machines, are down for some reason and one MUST supply the number to the network. However, that doesn't happen too often these days on modern and well maintained networks because there usually is more than one nameserver available, and if one is down another can often be found by the network when it needs a name translated

Let us consider as a typical example the internet address

www.techcorps.org

which is actually the name of the IP address 160.36.28.57. How did the people who run the Internet come up with that name, and how did we know that www.techcorps.org is really a nickname for 160.36.28.57?

The symbolic name of a site is determined by a naming standard known as the Domain Name System or DNS. Each field in a symbolic address corresponds to a single domain. The first field is a host name, which identifies a single computer. The last field is a top level domain. In between are things like department names, organization names, and so on in order of increasing generality.

For example, www.techcorps.org is the address for the main server of Tech Corps, which explains the ".techcorps" in the name, and this is an organization, which explains the ".org" part of the name. When we want to be precise we will term a sequence like 160.36.28.37 an IP address, while we will call the corresponding name csep10.phys.utk.edu the DNS name for the computer with IP address 160.36.28.37. However, in many instances we will use the term "IP address" to refer loosely to either the number or the name.

Some top level domains in the US and some sample addresses are listed in the preceding figure. Addresses of sites outside the US end with a two-letter country code; some common country codes are also listed in this figure.

f you wish, you may use this exercise to add links to your homepage that will aid in using electronic mail by providing ways to find addresses, and by providing on-line assistance for topics related to email. In the process, the exercise introduces the important technique of creating "lists" in HTML. In particular, you will see how to create numbered lists, where items in the list are numbered sequentially, and un-numbered lists, where items are not numbered, but each is preceded by a "bullet" symbol. You will also see how to "nest" one list as a sublist of another.