The nitelny program intended for reproduction of such movie — a flash player. Similar operations are performed also in case of detection on the page of the link to the sound file.
It is important to understand that all “not text” elements of the document are stored in separate files, and directly at the HTML-file there are only commands of a challenge of these elements. Therefore if some of such files will be unavailable or “unclear” to the browser, in the document on site of the absent element there will be “hole” (fig. 1.2).
Fig. 1.2. An example of the page with the absent “not text” element
For some types of “not text” elements own icon which is displayed by the browser for substitution of the absent element is provided. Having looked at such icon, the skilled visitor will understand directly, “what in soup is not enough”. For example, in fig. 1.3 the page for which the browser on could find the requested flash movie is shown.
8 Searching on the Internet
3 The Peter and Paul Fortress – Microsoft Int. M TH
Editing Type Selected Ce J file> Links> J O
Fig. 1.3. For some absent “not text” elements the icon deputy is provided
So, once again we will emphasize that the page uploaded by the browser may contain not only text information, but also other elements capable to draw attention of the visitor in itself. Let’s tell if the page is devoted to the report on a football match, then a certain visitor can become interested only in photos and completely ignore the text with comments. It is possible to assume that when searching such visitor will try to find in the Internet first of all pages with “football” illustrations or directly files of pictures.
Considering keen interest of visitors to Internetakmul-timediyny materials, owners of many web sites turned the ownerships into authentic “warehouses” of such resources. In particular, specialized storages of musical files in MP3 format enjoy wide popularity. In more detail on such websites it is told about features of searching in a chapter 3.
Probably the expression “resources of the Internet” mentioned above is familiar to the reader. It is usually used for designation of all information provided in the Internet irrespective of its type and a format. That is both HTML-files, and graphic files belong to resources,
1. Information representation on the Internet 9
and files of other types, to which registered access to visitors of the Network. It is apparent that the technology of searching depends on a type of a required resource. However availability on the Internet of a single system of addressing substantially smooths these distinctions.
Addressing on the Internet
If you want to communicate to someone in “usual”, not computer, lives, then you will need the address. As a rule, the addresses are organized in the form of a hierarchical system:
■ country ^ city ^ street ^ house ^ apartment or
■ telephone country code ^ city code ^ telephone number of the subscriber.
B the Internet is received the hierarchical system of the addresses too, it has five levels. Levels are called domains (areas). The top-most, root domain of a zerolevel is WWW in general. The most bottom (fourth) level corresponds to the concrete computer connected to the Network.
Rules of addressing are defined by the so-called network protocol, or the IP protocol (IP — Internet Protocol).
The network protocol is a rule set and the requirements defining a data interchange order on the Network. It is considered by development of the computer programs and engineering devices providing drive and data reception to Networks. If the program or the device meets the requirements of the network protocol, say that it supports this protocol.
By the name of the protocol the addresses used on the Internet began to call IP addresses. Each IP address is unique and consists of four numbers (from about to 255 inclusively) divided by points. For example, the combination at random selected 18.104.22.168 quite can be the IP address of some real computer.
The number extreme at the left corresponds to the highest level, and extreme on the right — to the concrete computer, that is the most bottom level of hierarchy (fig. 1.4).
Lack of the IP address is that the digital code is difficult for storage and perception by visitors of the Network. This shortcoming was corrected very simply: replaced the digital code with character designations (that is more or less intelligent words or abbreviations). Let’s tell, a combination of my. group. spb.ca — an example of a character equivalent of a digital IP address. The IP address presented in the character form is called domain, a name.
10 Searching on the Internet
Fig. 1.4. Structure of a LP address
Unlike the IP address, hierarchy levels in domain name are located upside-down: to the top-most level (the domain of a zerolevel) there corresponds the point extreme on the right. The string standing to the left of it (in the stated above example — ru) is a name of a top-level domain and so on. An extreme left-hand part of domain name corresponds the computer name connected to the Network.
As a rule, the root domain (which is designated by the most right point) when writing domain name is not specified.
One more essential difference of domain name from the IP address consists that it shall not consist of four parts. You can meet domain name of the computer which supports three, five or even six parts. For example, domain name of a website of St. Petersburg publishing house — www. piter. com.
The composition of parts of domain name depends on a logical structure of that domain to which the computer (fig. 1.5) belongs.
Control of use of domain names is exercised by express service of the Network — Service of domain names (Domain Name Service, DNS). A primal problem of DNS — it is correct to compare to domain name of the computer its IP address. The procedure of comparison the IP address of domain name is usually called permission (resolving).
Search of the IP address in domain name is executed by so-called name servers (nameserver), is more often than them call DNS servers. Each DNS server stores information on several domains. So-called root servers store information on name servers for all top-level domains (top-level domains, TLD). Now there are 13 root servers, and all of them are located in the USA. If the DNS server of any level cannot self-contained razre-
1. Information representation on the Internet 11
to sew domain name, he applies for information to the higher DNS server. It repeats until the response is not found or will not be clarified yet that the specified address does not exist.
www. set. spb.ca
red. set. spb.ca
Fig. 1.5. The domain name can have different length
Let’s consider operation of DNS on an example. Let’s assume, working at the computer with the name comp. set. spb.ca, you want to visit the website to the address www. piter. com.
First of all your computer will send request for determination of the IP address of the website www. piter. com to the DNS server of the organization which supports the domain set. Let’s say on this DNS server the required IP address is absent. B such case the DNS server will send request to the higher server supporting the domain spb. If here it is possible to find nothing, the request is sent DNS серверудомена by.ca. This server too can “not know”, where to look for the necessary website, but he perfectly “knows” where there is a domain. com DNS server, to it it also will readdress your request. The server of the domain. com contains information on all subdomains, including on the domain piter. com. But about where in the domain piter there is a Web server of www. piter. com, he the nobility is not obliged. Therefore he sends request to the DNS server of St. Petersburg publishing house and already receives the IP address of a required Web server from it. This address is passed on a line-up of requests back and eventually enters on your computer. And only after that the browser addresses directly on the specified IP address for obtaining the necessary web page (fig. 1.6).
B the present practically each civilized country has the top-level domain (today there are about 300 such domains). The name of the domain corresponds to the international two-letter country code in ISO. For example, a name of the domain of Russia — ru, Ukraine — ua, the USA — us. The single exception — Great Britain. Its ISO code — gb, however as domain name are used the letters uk.
12 Searching on the Internet
Domain DNS server ^_
Domain DNS server ‘set
Domain DNS server
Request of v www. piter. com
Domain piter. com DNS server
comp. set. spb.ca www. piter. com of Fig. 1.6. Simplified diagram of searching IP address
In top-level domains additional division is usually used. For example, geographical domains are usually sectioned according to the administrative structure of this community. Creation of domains of the second level for the large cities is rather widespread in Russia. So, the domain of St. Petersburg is called spb.ca, the domain of Moscow — msk.ca.
Along with the geographical principle of education of domains the top-level domains which remained since those times when boundaries of the Internet “matched” boundaries of the USA are widely used. Of nikhyavlyatsya by the most known:.com are the commercial organizations,.net are the network organizations,.edu are educational institutions,.org are the public (non-profit) organizations,.mil are organizations of the U. S. Department of Defense.