web browser

1. Unit 3 3 Introduction to Browser A web browser is a software application which enables a user to display and interact with text, images, videos, music and other information typically located on a Web page at a website on the World Wide Web or a Local Area Network. Text and images on a Web page can contain hyperlinks to other Web pages at the same or different website. Web browsers format HTML information for display, so the appearance of a Web page may differ between browsers. Some of the Web browsers available for personal computers include Internet Explorer, Mozilla Firefox, Safari, and Opera in order of descending popularity. Web browsers are the most commonly used type of HTTP user agent. Although browsers are typically used to access the World Wide Web, they can also be used to access information provided by Web servers in private Networks or content in File Systems. Web browsers communicate with Web servers primarily using HTTP (Hypertext Transfer Protocol) to fetch Web pages. HTTP allows Web browsers to submit information to Web servers as well as fetch Web pages from them. 3.1. Internet Explorer Windows Internet Explorer (formerly Microsoft Internet Explorer abbreviated as MSIE), commonly abbreviated as IE, is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems starting in 1995. It has been the most widely used web browser since 1999. 3.1.1 Features of Internet Explorer Internet Explorer has been designed to view the broadest range of web pages and to provide certain features within the operating system including Microsoft Update. Some of its features are as follows: 1. Standard support Internet Explorer almost fully support HTML, CSS XML implementation techniques. 2. Usability and Accessibility Internet Explorer makes use of the accessibility framework provided in Windows. Internet Explorer is also a user interface for FTP. Recent versions feature pop – up blocking and tabbed browsing. 3. Cache Internet Explorer caches visited content in the Temporary Internet Files folder to allow quicker access to previously visited pages. 4. Security Internet Explorer uses a zone – based security framework that groups sites based on certain conditions, including whether it is an intranet or internet – based site. Security restrictions are applied on a per – zone basis, all the sites in a zone are subject to the restrictions. https://www.ThesisScientist.com

2. 5. Group Policy Internet Explorer is fully configurable using Group Policy (feature thatprovides centralized management and configuration of computers and remote users). Administrators of Windows Server Domains (a logical group of computers running versions of the Microsoft Windows operating system that share a central directory database) can apply and enforce a variety of settings that affect the user interface (such as disabling menu items and individual configuration options), as well as underlying security features such as downloading of files, zone configuration, per – site settings, etc. 3.1.2 Criticisms to Internet Explorer Internet Explorer has been subject to many criticisms. Most of the criticism concerns its security architecture and its degree of support of open standards. Much criticism of Internet Explorer is related to concerns about security. Much of the spy ware, ad ware, and computer viruses across the Internet are made possible by exploitable bugs and flaws in the security of Internet Explorer, sometimes requiring nothing more than viewing of a malicious web page in order to install them. 3.2.1 Netscape Navigator Netscape Navigator, also known as Netscape, is a proprietary (proprietary software is a term for computer software with restrictions on use or private modification, or with restrictions judged to be excessive on copying or publishing of modified or unmodified versions) web browser that was popular during the 1990s. It is a “closed –source”, “non –free” web browser. Initially it was known as Mosaic browser and was a paid commercial web browser. But later in 1994 it was decided to make it freely available for all non – commercial users. During development, the Netscape browser was known by the code name Mozilla. The Mozilla name was used as the user – Agent in HTTP requests by the browser. Other web browsers claimed to be compatible with Netscape’s extensions to HTML. Mozilla is now a generic name for matters related to the open source successor to Netscape Communicator. 3.2.1.1 The Rise of Netscape When the consumer Internet revolution arrived in the mid-to-late 1990s, Netscape was well positioned to take advantage of it. With a good mix of features and an attractive licensing scheme that allowed free use for non-commercial purposes, the Netscape browser soon became the de facto (in fact / in practice) standard, particularly on the Windows platform. Internet service providers and computer magazine publishers helped make Navigator readily available. An important innovation that Netscape introduced in 1994 was the on-the-fly display of web pages, where text and graphics appeared on the screen as the web page downloaded. Earlier web browsers would not display a page until all graphics on it had been loaded over the network connection; this often made a user stare at a blank page for as long as several minutes. With Netscape, people using dial-up connections could begin reading the text of a webpage within seconds of entering a web address, even before the rest of the text and graphics had finished downloading. This made the web much more tolerable to the average user. 3.2.1.3 Fall of Netscape https://www.ThesisScientist.com

3. Microsoft saw Netscape's success as a clear threat to the dominant status of the Microsoft Windows operating system. It began a wide-reaching campaign to establish control over the browser market. The resulting battle between the two companies became known as the browser wars. Netscape Navigator 3.0 came in two versions, Standard Edition and Gold Edition. The latter consisted of the Navigator browser with mail and news readers and a web page WYSIWYG composition tool integrated into it. The extra functionality only made the software program larger, slower, and more prone to crashes, and the decision to integrate all these features together was widely criticized. By the end of the decade, Netscape's web browser had unquestionably lost its former dominance on the Windows platform. 3.2.1.4 Criticisms to Netscape Netscape Navigator has mostly been criticized for implementing non-standard HTML mark-up extensions such as the BLINK tag, which is sometimes referred to as a symbol for Netscape's urge to develop extensions not standardized by the W3C (The World Wide Web Consortium (W3C) is the main international standards organization for the World Wide Web (abbreviated WWW or W3). It is arranged as a consortium where member organizations maintain full-time staff for the purpose of working together in the development of standards for the W3), and even mentioned in the fictional Book of Mozilla. Netscape has also been criticized for following actual web standards poorly, often lagging behind or supporting them very poorly or even incorrectly. This criticism wasn't very loud during the days of its popularity as web designers then often simply developed for Netscape Navigator, but came to be an increasing annoyance to web designers who wish to provide backward compatibility, most often with Netscape Navigator 4 and Netscape Communicator, to their web sites. Today, many web masters simply do not choose to support these old versions, due to their extremely small market share and lack of standardization. However, Netscape's own contributions to the web of this sort haven’t always been of frustration to web developers. 3.4 The Web provides a means of accessing an enormous collection of information, including text, graphics, videos, movies, and so on. The information on the web can be accessed in a non – linear and experimental fashion. The user can “jump” from topic to topic via hyperlinks, without the need of sequential navigation. This nonlinear approach to information gathering, or browsing, is sometimes referred to as “surfing the Web”. 3.5. Web Terminologies Some of the commonly used terms associated with Web surfings are as follows: 1.Page or Web Page. A file that can be read over the World Wide Web. The global collection of documents associated with and accessible via the World Wide Web. Coast – to – Coast surfing https://www.ThesisScientist.com

4. 2Hyperlink. A string of clickable text or a clickable graphic that points to another Web page or document. When the hyperlink is selected, another Web page is requested, retrieved, and rendered by the browser. 3Hypertext. Web pages that have hyperlinks to other pages. More generally, any text having nonlinear links to other text. 4Browser. A software tool used to view Web pages, read email, and read newsgroups, among other things. Browsers are also called Web clients. 5Multimedia. Information in the form of graphics, audio, video, or movies. A multimedia document contains a media element other than just plain text. 6Hypermedia. Media with links and navigational tools. 7Uniform Resource Locator. A string of characters that specify the address of a Web page. 8Surfer. A person who spends time exploring the World Wide Web. 9Web Presentation. A collection of associated and hyperlinked Web pages. Usually, there is an underlying theme to the pages. For example, a Web presentation for a company may describe facts about the company, its employees, its products, and the method for ordering the products on – line. 10Webmaster. A person who maintains, creates, and manages a Web presentation, often for a business, organization, or university. This person usually “signs” Web pages, so that questions and comments can be sent to them. 11Web manager. Synonym for Webmaster. 12Web site. An entity on the Internet that publishes Web pages. A Web site typically has a computer serving Web pages, whereas a Web presentation is the actual Web pages themselves. For example, www.bsnl.com is the name of a Web site, whereas 13Web server. A computer that satisfies users requests for Web pages. 14Mirror site. A site that contains a duplicate copy of a Web presentation from another site. If a Web presentation is extremely popular, other sites may be used to mirror the original presentation; i.e., they contain the same information as the original site. This allows the load on the Web server and the network to be distributed. If one server is down, a mirror site can be tried. If several mirror sites exist, it is a good idea to try the closest to the user first. 3.6 Uniform Resource Locator (URL) A URL or Uniform Resource Locator may be defined as the Web address that uniquely identifies a computer on Internet. A URL points to three issues as: 1. How can the page be accessed? 2. Where can the page be found? 3. What is the file name corresponding to the page? The format of a URL is as follows: how : // where / what For example, consider a URL as follows: http : // www.google.org / home.php https://www.ThesisScientist.com

5. Here, http –defines the protocol or scheme by which to access the page. HTTP (Hypertext Transfer Protocol) is the set of rules by which an HTML document is transferred over the Web. www.google.org –identifies the domain name of the computer where the page resides. The domain name tells the browser on which computer to find the Web page. home.php –provides the local name uniquely identifying the specific page. This example demonstrates that the URL consists of a protocol, a Web server’s domain name, and a file name. 3.7 Hypertext Markup Language HTML or Hypertext Markup Language is a markup language used to create documents on the World Wide Web. It defines the structure and layout of a Web document by using a variety of tags and attributes. A markup language is a set of interpretations to text that describe how it is to be structured, laid out, or formatted. The tags are predefined combinations of characters enclosed between < (less than) and > (greater than) characters. These < and > characters are called as angular brackets. The tags are embedded within the text of a file, and they indicate how the text is to be interpreted and displayed by the browser. How a Web page looks when displayed depends on three things: 1. The HTML tags used. 2. The specific browser rendering the page. 3. The user’s system and monitor. HTML tags do not define exactly how the Web page is supposed to look; rather, the tags describe how the elements of the page, such as headings, lists, paragraphs, and so on, are to be used. HTML is not case sensitive, i.e., the tags either in uppercase like <HTML> or the tags in lower case like <html> means the same as <Html>. 3.7.1 HTML Tag Syntax The basic form of HTML tags can be written abstractly as: <TAG ATTRIB1= “VALUE1” ATTRIB2= “VALUE2”> item to be formatted</TAG> where ATTRIB means attribute.TAG means any HTML tag. The number of attributes varies from tag to tag. An attribute can have several values to be selected. Every HTML tag has a closing tag or an ending tag as </TAG>. However, some of the tags might work without the ending tags. https://www.ThesisScientist.com

6. HTML is not case sensitive, i.e., the tags either in uppercase like <HTML> or the tags in lower case like <html> means the same as <Html>. 3.8.2 HTML Document Creation To produce an HTML document, a text editor is needed to create an ASCII file with an extension .html or .htm. An HTML document is created using the <HTML> and </HTML> tags in a text editor, that may be a notepad or the word pad. Every HTML document has two parts: a head and a body. These parts can be distinguished by using following tags: <HEAD> and </HEAD> for the head part. <BODY> and </BODY> for the body part. A title tag, <TITLE>, is contained within the head of the document to provide a title for the document, with a corresponding closing tag </TITLE>. The title should provide a concise description of the page, since the title is prominently displayed in the browser window’s Title bar when the page is being viewed. For example, consider the following HTML document, “sample . html”: <HTML> <HEAD> <TITLE> Internet Fundamentals</TITLE> </HEAD> <BODY> This is my first Web page. Welcome to the Web World. This is an example of Internet Fundamentals. </BODY> </HTML> After creating the HTML document as mentioned above, the page will be displayed as follows: https://www.ThesisScientist.com

7. 3.9 In order to view the pages on the Web, the user will need to install them on a Web server. A Web server is a program, located on a computer with Internet access that responds to a browser’s request for a URL. That is, a Web server meets the demands of user’s by supplying or serving them the Web pages requested. A Web server is accessible to the user in many ways as many ISPs include space on their computer that runs the Web server as part of the basic set of services covered by them. The systems administrator responsible for the server can usually provide the user with site – specific details for publishing the Web pages. Other than the ISPs, there are many Web sites providing the facility for free Web hosting for Web page installations. 3.9.1 Basic Principle The various principles towards the Web page installations are as follows: 1. User need to have Web pages to publish. 2. A Web server where the files can be placed must be available to the user and the user need to learn the steps to put files in place, either to create them in place or copy them into place after the user develop and test them. 3. The permissions on the files need to be set so that any user anywhere can read them. Such file permissions are often referred to as world readable. The details of exactly how these steps are performed vary from platform to platform. Some of the examples of Web servers providing service for Web publishing are: Web Page Installation https://www.ThesisScientist.com

8. 3.10 Web Page Setup Besides the basic tags used for HTML document creation, there are many other tags provided for proper Web page setup. These may be explained as follows: 3.10.1 Head Tag The head tag, <HEAD>, has no attributes. However, several tags can be included inside it. The most important is the title tag <TITLE> used to determine the title of the Web page. Some other tags used are as follows: 3.10.1.1 Basefont Tag The basefont tag, <BASEFONT>, defines the font size to be used in the HTML document and may be included in the head of the document. It can also be used in other locations of an HTML document. It ranges from the value 1 to 7. The default font size is 3. It can be used as shown below: <HEAD> <BASEFONT SIZE = ”4”> or </HEAD> 3.10.1.2 Base Tag The base tag, <BASE>, is useful for setting some global parameters of an HTML document and may be included in the head of the document. A global parameter is an attribute that has an effect on the entire document. It provides a relative URL for a document. It can be used as follows: <HEAD> <TITLE>Water Sports to Die for</TITLE> <BASE HREF = “http://www.fishing.com/BOATS/outboard.html”> </HEAD> Here “http://www.fishing.com/BOATS/outboard.html” be the relative URL for a document with absolute URL http://www.paloalto.gov/entertainment/water.html. 3.10.1.3 Meta Tag The second most widely used tag inside the head part is the meta tags. This tag is used to include additional information about a document and can be used to pass additional http://www.hitech-on-web.com/Free_Web_Templates.asp http://www.aspwebserver.com/ http://www.hostmonster.com/ http://www.startlogic.com/startlogic/index.bml http://www.ipower.com/ipower/index.bml http://www.bluehost.com/ http://www.hostican.com/ <HEAD> <BASEFONT SIZE = “+1”> </HEAD> https://www.ThesisScientist.com

9. information to a browser. There is no ending tag for <META> tag and a document can have multiple <META> tags. Its attributes are NAME, CONTENT, and HTTP – EQUIV. The value of the attribute CONTENT can be keywords related to the document, say three to five. If someone is searching for a particular topic, the page may be returned if one or more keywords match their search. For example, <HEAD> <META NAME = “keywords” CONTENT = “internet, HTML, basic tags, Meta tag”> </HEAD> 3.10.2 HTML and Colors There are two ways of defining colours in HTML documents. One involves straightforward color names, such as blue, green, orange, red, and yellow. Many browsers have a list of predefined color names. However, since different browsers have different lists and since the definitions of individual colors may vary from browser to browser, the color numbering scheme is recommended to be used. The numbering scheme uses three two digit hexadecimal numbers. Each of the two digits specifies the amount of one of the three primary colors: Red, Green and Blue respectively i.e., RGB. Some Colors and their Corresponding Hexadecimal Representations are as follows: Black #000000 Blue #0000FF Gold #FFD700 Gray #808080 Maroon #800000 Violet #EE82EE 3.10.3 Body Tag The body is the main part of every HTML document. The text and HTML code that goes between the body beginning and ending tags is rendered and displayed in the document area of the browser’s window. The <BODY> tag has a number of useful attributes used to define global parameters. 3.10.3.1 Text Color The TEXT attribute is used to change the default text color for an entire document. For example, <BODY TEXT = “#800000”> for maroon text color. 3.10.3.2 Background Color and Tiling Two attributes to the <BODY> tag that improves the appearance of a Web page are BGCOLOR and BACKGROUND. Orange Purple Silver White Yellow Red #FFA500 #800080 #C0C0C0 #FFFFFF #FFFF00 #FF0000 https://www.ThesisScientist.com

10. BGCOLOR attribute is used to set the background of an HTML document to a single color. For example, <BODY BGCOLOR = “#0000FF”> or to set the back ground color to blue. BACKGROUND attribute is used to place an image in the background of an HTML document. If postage – stamp – sized image is used, it will appear repetitively creating tiles on the document. This is called tiling. For example, <BODY BACKGROUND = “marble.jpg”> 3.10.3.3 Hyperlink Colors Three attributes are used for changing the color of a hyperlink, where the color depends on the current state of the hyperlink. The three possible states are: unvisited, visited, and currently thinking of visiting. These are defined as: LINK - Unvisited hyperlinks. The color value assigned to LINK sets the color for all unvisited hyperlinks in the document. VLINK - Visited Hyperlinks. The color value assigned to VLINK sets the color for all the visited hyperlinks in the document. ALINK - Active Hyperlink. A hyperlink the user is thinking of visiting. The color value assigned to ALINK sets the color of the hyperlink that the user has mouse over and depressed the mouse button on. 3.10.4 HTML Font Colors The <FONT> attribute allows changing the color of any portion of text using COLOR attribute. <FONT COLOR = “#0000FF”> Internet Fundamentals </FONT> This will change the color of the text portion Internet Fundamentals to blue. 3.10.5 HTML Font Size The <FONT> attribute allows to change the size of an individual part of a document using SIZE attribute. <FONT SIZE = “+3”> W </FONT> elcome my friend. It will display the text as: Welcome my friend. <BODY BGCOLOR = “blue”> https://www.ThesisScientist.com

11. 3.10.6 HTML Font Face FACE attribute in <FONT> tag allows the user to specify a particular font type. For example, <FONT FACE = “Century Gothic”>New font type</FONT> It will display the text as: New font type 3.10.7 HTML Comments The HTML document does not employee any <COMMENT> tag for placing any comment in the document. Instead, it is the set of symbols < ! - -for the beginning and - - > for the ending tag. For example, < ! - - This is a comment - - > The browser will not interpret the text between the pair of dashes. 3.11 HTML Formatting and Hyperlink Creation (refer to Book: In – line / On – line fundamentals of the Internet and the World Wide Web by Raymond Greenlaw) (Page: 84 - 95) The various tags used for formatting and hyperlink creation in an HTML document are as follows: Paragraph tag. <P></P> Heading Tags. <H1> through <H6> Anchor Tag. <A HREF = “URL”></A> Image Tag.<IMG SRC = “marble.jpg” HEIGHT = “5” WIDTH = “5” ALT = “Under Construction”> Clickable Image Hyperlinks Mailto Hyperlinks. <A HREF = “mailto: example@gmail.com”>Contact Greenlaw</A> Intradocument Linking. <A NAME = “top”><A HREF = “#bottom”><H3>Beautiful Blueberry</H3></A></A> …… ….. …… <A NAME = “bottom”><A HREF = “#top”><H3>Cherry</H3></A></A> …… ….. https://www.ThesisScientist.com

12. …… 3.12 FrontPage Express Microsoft FrontPage (later full name Microsoft Office FrontPage) was a WYSIWYG (What You See Is What you Get) HTML editor and web site administration tool from Microsoft for the Microsoft Windows line of operating systems. It was part of Microsoft Office application suite from 1997 to 2003. As a WYSIWYG editor, FrontPage is designed to hide the details of pages' HTML code from the user, making it possible for novices to easily create web pages and sites. 3.12.1 Features Some features that are part of the last version of FrontPage include: Help navigating through your site, and seeing your file structure, visually Built-in features for HTML, CSS, and JavaScript (partial) Built in image editor (MS Image Composer) Point-and-click functionality for common tools, like mouse over, e-mail forms, and hit counts Simple to use with previous knowledge of Office products Integrated data display with Office products like Access and Excel Support for CSS-based themes (like ASP.NET master pages) When you change the URL of a page, all the links to that page are dynamically changed Task-assignment for team projects Content is editable from anywhere with FrontPage (password is needed) Support for rich clipboard data import (i.e. copy/pasting data from Internet Explorer into FrontPage 2003 will automatically download media resources such as images and save them locally) Built-in support for automated web templates that includes automatically generated multi-level navigation system. FrontPage 2003 introduced support for Dreamweaver compatible web templates, dubbed "Dynamic Web Templates".              4 Application layer protocols 4.1 File Transfer Protocol (FTP):is used for interactive file transfer.FTP uses TCP as its transport. It has two well-known ports: • FTP data port is 20 • FTP control port is 21 The commands and status information are sent over the control port; the requested files, etc. Are transferred on the separate data port. The standard for FTP, illustrates a typical FTP set-up, as shown in Fig 4.1 Fig 4.1 of Typical FTP implementation https://www.ThesisScientist.com

13. The user types commands such as “get /tmp/x” into the part of the client program that handles the user interface. This part of the program communicates the user’s instructions to the “protocol interpreter” (PI) – a separate part of the program that sends FTP protocol commands to the server, and interprets responses from the server. The PI also controls the “data transfer process” (DTP) component, instructing it as to when to connect to the server, which IP address and port number to connect on, which files to transfer, etc. The actual file data, directory listings, and so on are transferred over the data connection. The FTP protocol can handle “third-party” transfers, from one remote server to another remote server, controlled by a client elsewhere as shown in Fig 4.2 below, which is why the control and data connections are separate. Fig 4.2 Client controls third-party transfer between two other machines How FTP connections are established A typical FTP session consists of two phases (Fig 2.8 below): 1. The user specifies to the client which server and port to connect to. The client establishes the control connection. The user logs in, negotiates any transfer parameters with the server (e.g. binary or text mode transfer), and sets any local parameters . 2. The user requests a file transfer (from server to client or vice versa). The data connection has to be established now. The procedure is: https://www.ThesisScientist.com

14. 2.1 the client opens an ephemeral port and listens on it, waiting for the server to connect. 2.2 The client sends the FTP PORT command, telling the server the number of the port the client is listening on. 2.3 the server connects to that port and the connection is established 2.4 the data are transferred over the connection 2.5 The connection is closed. Phase 2 is repeated for each transfer , using a new data connection each time. Fig 4.3 show two phases of a typical “active mode” FTP session The way the data connection is established causes problems when the connection is through a firewall, from an internal client to an external server. The data connection is established from outside the firewall (the server) to inside (the client) on a more or less random port (:1068 here). The firewall doesn’t know whether this is the second phase of a genuine FTP session that an internal client established or an outside hacker trying to break in and consequently many firewalls block it. To overcome this problem, the FTP server can operate in passive mode (as opposed to active mode Fig 4.4 of a “passive mode” FTP session https://www.ThesisScientist.com

15. 4.2 Telnet:- “Telnet” stands for “telecommunications networkprotocol.” telnet is one of the oldest Internet applications. It lets you connect your “terminal” to a remote host over the network. telnet is typically used to “remote login” from your own PC to another elsewhere on the network. Remote login like this lets you use applications on the remote system as shown if fig below It provides a text-only connection, usually to a “command-line prompt” like a UNIX shell, or Windows Command/DOS prompt, as though you were sitting at a terminal wired directly to a serial port on the remote machine. This was how networked computing was performed for years. Telnet is a client/server application. The client takes the characters you input at the keyboard, sends them to the server, and prints whatever output the server sends back. The server does more work: it passes the input characters from the client to a shell process, which interprets them as commands; then it reads the command output and sends it back to the client, to be printed on your terminal’s output device. Fig 4.5 of using telnet to log on remote host https://www.ThesisScientist.com

16. Fig 4.6 below shows the typical arrangement of the Telnet client and server. 1. The Telnet client interacts with both the user at the terminal and the TCP/IP protocols. Normally everything we type is sent across the TCP connection, and everything received from the connection is output to our terminal. 2. The Telnet server often deals with what's called a pseudo-terminal device, at least under Unix systems. This makes it appear to the login shell that's invoked on the server, and to any programs run by the login shell, that they're talking to a terminal device. 3. Only a single TCP connection is used. Since there are times when the Telnet client must talk to the Telnet server (and vice versa) there needs to be some way to delineate commands that are sent across the connection, versus user data. 4. We show dashed boxes in Figure to note that the terminal and pseudo terminal drivers, along with the TCP/IP implementation, are normally part of the operating system kernel. The Telnet client and server, however, are often user applications. 5. We show the login shell on the server host to reiterate that we have to login to the server. We must have an account on that system to login to it, using either Telnet or Rlogin. 4.3 SMTP: Simple mail transfer protocol Internet e-mail systems use TCP as their transport layer and are client/server applications. On a typical LAN, Internet e-mail involves three separate steps: 1. Your client sends messages using SMTP (Simple Mail Transfer Protocol) 2. Your client receives messages from the server using POP3 (Post Office Protocol, version 3) 3. Your server (or your ISP’s server) sends and receives messages from other servers using SMTP. https://www.ThesisScientist.com

17. That is, SMTP is used everywhere except by clients retrieving their mail from their own (or their ISP’s) server. All e-mail over the Internet is sent using SMTP. Even if your mail server is Microsoft Exchange or Lotus, it uses SMTP to send to remote sites, and to receive from them Fig 4.7 of Email protocols 4.4 DNS: (Domain Name Service) a) DNS Specifies the name syntax and rules for delegating authority over names Specifies the implementation of distributed computing system that efficiently map names to addresses. b) DNS Syntax Set of labels separated by delimiter character (period) Example: ecs.syr.edu syr.edu is also a domain The top-level domain is edu c) Original Top-Level Domains Fig 2.13 show domain name https://www.ThesisScientist.com

18. d) Mapping Domain Names to Addresses • Name server: supplies name-to-address translation • Client: uses one or more name servers when translating a name • DNS uses a set of on-line servers • Servers arranged in tree • Given server can handle entire subtree. For example, ECS manages domain names within the ecs.syr.edu domain. Fig 2.14 show domain tree structure https://www.ThesisScientist.com