Web Browser

•A Web browser is a software application which enables a user to display and interact with text, images, videos, music, games and other information typically located on a Web page at a Web site on the World Wide Web.

•Text and images on a Web page can contain hyperlinks to other Web pages at the same or different Web site.

•Web browsers allow a user to quickly and easily access information provided on many Web pages at many Web sites by traversing these links.

•Web browsers format HTML information for display, so the appearance of a Web page may differ between browsers.

Internet Explorer, Opera, Mozilla Firefox, Google Chrome



Brower - Server communications

§Type the URL in the web browser. If the URL contains a domain name, the browser first connects to a domain name server and retrieves the corresponding IP address for the web server.

§The web browser connects to the web server and sends an HTTP request (via the protocol stack) for the desired web page.

§The web server receives the request and checks for the desired page. If the page exists, the web server sends it. If the server cannot find the requested page, it will send an HTTP 404 error message. (404 means 'Page Not Found')

§The web browser receives the page back and the connection is closed.

§The browser then parses through the page and looks for other page elements it needs to complete the web page. These usually include images, applets, etc.

§For each element needed, the browser makes additional connections and HTTP requests to the server for each element.

When the browser has finished loading all images, applets, etc. the page will be completely loaded in the browser window.

Universal Resource Location (URL)

Domain Names and Address Resolution

•To connect from computer A to computer B an IP address is used

•Usually people do not use IP addresses directly to access other computers

Domain Name Service or DNS distributed database is used to map names to IP addresses

DNS: Domain Name Service

•DNS is a distributed database of computers’ names and their corresponding IP addresses

•DNS servers are used to host the DNS database and software to use it

•Since the DNS database is too large to be stored on any specific computer only a subset of the database is usually kept on any DNS server

Name à IP Address

•The job of the DNS server is to map the name to an IP address

•

If the DNS server can not resolve the domain request (because it does not have it in its part of the database) then it redirects the request to a different DNS server

Hypertext Transfer Protocol (HTTP)

•HTTP is a request/response standard of a client and a server.

•The client making a HTTP request—using a web browser, spider, or other end-user tool—is referred to as the user agent.

•The responding server—which stores or creates resources such as HTML files and images—is called the origin server.

•HTTP is a connection-less text based protocol.

Hypertext Markup Language (HTML)

•It provides a means to describe the structure of text-based information in a document—by denoting certain text as links, headings, paragraphs, lists, etc.—and to supplement that text with interactive forms, embedded images, and other objects.

•HTML is written in the form of "tags" that are surrounded by angle brackets.

•HTML can also describe, to some degree, the appearance and semantics of a document, and can include embedded scripting language code (such as JavaScript) that can affect the behavior of Web browsers and other HTML processors.