INTERNET APPLICATIONS PREPARED BY: V.C.DANDWANI OVERVIEW 6.1 DNS 6.2 WWW 6.3 ELECTRONIC MAIL 6.4 INTRANET 6.5 WIRELESS NETWORKS 6.6 HOME NETWORKS 6.1 DNS-DOMAIN NAME SYSTEM The domain name system is usually used to translate a host name into an IP address. Domain names comprise a hierarchy so that names are unique, yet easy to remember. The Domain Name System makes it possible to assign domain names to groups of Internet resources and users in a meaningful way, independent of each entity's physical location.domain names The Domain Name System distributes the responsibility of assigning domain names and mapping those names to IP addresses. A DNS program works like this - every time a domain name is typed in a browser it is automatically passed on to a DNS server, which translates the name into its corresponding IP address.DNS server Resolver and name server An application program on a host accesses the domain system through a DNS client, called the resolver. Resolver contacts DNS server, called name server. DNS server returns IP address to resolver which passes the IP address to application. Reverse lookups are also possible, i.e., find the hostname given an IP address. DNS hierarchy can be represented by a tree. Labels are connected together with a dot notation, ".", and a DNS name containing multiple labels represents its path along the tree to the root. Root and top-level domains are administered by an Internet central name registration authority (ICANN) Below top-level domain, administration of name space is delegated to organizations Each organization can delegate further DNS HIERARCHY Domain name system Each node in the DNS tree represents a DNS name Each branch below a node is a DNS domain. DNS domain can contain hosts or other domains (sub domains) Example: DNS domains are., edu, virginia.edu, cs.virginia.edu Top-level domains Three types of top-level domains: 1)Organizational: 3-character code indicates the function of the organization Used primarily within the US Examples: gov, mil, edu, org, com, net 2)Geographical: 2-character country or region code Examples: us, va, jp, de 3)Reverse domains: A special domain (in-addr.arpa) used for IP address-to- name mapping Address mapping Need protocols to create a mapping between physical and logical address.IP packets use logical(host to host) address. These packets need to be encapsulated in frame, which needs physical address(node to node).ARP-ADDRESS RESOLUTION PROTOCOL,RARP-REVERSE ADDRESS RESOLUTION PROTOCOL(mapping physical address to logical address) Packets pass through physical networks to reach these hosts and routers. At the physical level, the hosts and routers are recognized by their physical addresses. physical address is a local address. it is so called because it is usually implemented in hardware.(eg.48bit MAC address in Ethernet protocol). The physical address and logical address are two different indentifiers.we need both because a physical network such as Ethernet can have two different protocols at the network layer such as IP and IPX(novell) at same time.Likewise,a packet at a network layer such as IP may pass through different physical networks such as Ethernet and local talk.(apple)\ We need to map a logical address to its corresponding physical address and vice versa. Two types: static mapping and dynamic mapping STATIC MAPPING: It involves in the creation of a table that associates a logical address with a physical address. This table is stored in each machine on the network. Each machine that knows, for example, IP address of another machine but not its physical address can look it up in the table. DYNAMIC MAPPING: In this each time a machine knows one of the two addresses(logical or physical),it can use a protocol to find the other one. 6.2 WWW-WORLD WIDE WEB WEB BROWSER A web browser or Internet browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web.software applicationWorld Wide Web An information resource is identified by a Uniform Resource Identifier (URI) and may be a web page, image, video, or other piece of content.Uniform Resource Identifierweb page Hyperlinks present in resources enable users to easily navigate their browsers to related resources. Hyperlinks Although browsers are primarily intended to access the World Wide Web, they can also be used to access information provided by Web servers in private networks or files in file systems. Some browsers can also be used to save information resources to file systems.Web serversprivate networksfile systemsfile systems The primary purpose of a web browser is to bring information resources to the user. This process begins when the user inputs a Uniform Resource Identifier (URI), for exampleinto the browser.Uniform Resource Identifier The prefix of the URI determines how the URI will be interpreted. The most commonly used kind of URI starts with http: and identifies a resource to be retrieved over the Hypertext Transfer Protocol (HTTP). Many browsers also support a variety of other prefixes, such as https: for HTTPS, ftp: for the File Transfer Protocol, and file: for local files. Hypertext Transfer ProtocolHTTPSFile Transfer Protocolfile: Prefixes that the web browser cannot directly handle are often handed off to another application entirely. For example, mailto: URIs are usually passed to the user's defaultapplication and news: URIs are passed to the user's default newsgroup reader. In the case of http, https, file, and others, once the resource has been retrieved the web browser will display it. HTML is passed to the browser's layout engine to be transformed from markup to an interactive document. Aside from HTML, web browsers can generally display any kind of content that can be part of a web page. HTMLlayout enginemarkup Most browsers can display images, audio, video, and XML files, and often have plug-ins to support Flash applications and Java applets.XMLplug-insFlash Java applets Upon encountering a file of an unsupported type or a file that is set up to be downloaded rather than displayed, the browser often prompts the user to save the file to disk or identify a program that can open it. HTML HTML, which stands for Hyper Text Markup Language, is the predominant markup language for web pages. HTML is the basic building- blocks of WebPages.markup languageweb pages HTML is written in the form of HTML elements consisting of tags, enclosed in angle brackets (like ), within the web page content.HTML elementsangle brackets HTML tags normally come in pairs like and. The first tag in a pair is the start tag, the second tag is the end tag (they are also called opening tags and closing tags). In between these tags web designers can add text, tables, images, etc. The purpose of a web browser is to read HTML documents and compose them into visual or audible web pages. The browser does not display the HTML tags, but uses the tags to interpret the content of the page.web browser HTML elements form the building blocks of all websites. HTML allows images and objects to be embedded and can be used to create interactive forms. images and objectsinteractive forms It provides a means to create structured documents by denoting structural semantics for text such as headings, paragraphs, lists, links, quotes and other items.structured documents semantics It can embed scripts in languages such as JavaScript which affect the behavior of HTML WebPages.scriptsJavaScript WEB SERVERS Web servers are computers that deliver (serves up) Web pages. Every Web server has an IP address and possibly a domain name. For example, if you enter the URLin your browser, this sends a request to the Web server whose domain name is pcwebopedia.com. The server then fetches the page named index.html and sends it to your browser.computersWeb pagesIP addressdomain nameURLbrowser Any computer can be turned into a Web server by installing server software and connecting the machine to the Internet.softwareInternet There are many Web server software applications, including public domain software from NCSA and Apache, and commercial packages from Microsoft, Netscape and others.MicrosoftNetscape 6.3 ELECTRONIC MAIL (1) functions ofsystem: Send mail electronically via the Internet Requires an account on a mail server and supporting software on your PC The username and password will allow you to access your account Allprograms allow you to Send, Compose, Reply, and Forward mail access to some form of " " (electronic mail) - as opposed to "snail mail" (traditional post office variety).is the most basic function of networks. It allows people on networks to communicate with each other and, through interconnected "gateways," to talk to people on other networks as well. exchanging information, communicating ideas, discussing issues, sharing files, and editing and reviewing manuscripts(the form of a literally work submitted for publication or hand written notes) (2) User agent A user agent is a client application implementing a network protocol used in communications within a clientserver distributed computing system. The term most notably refers to applications that access the World Wide Web.clientnetwork protocolclientserverWorld Wide Web Back then, the Internet was (an actually still is) completely text-based, and to navigate the text, text commands needed to be typed into a keyboard. Soon tools were developed to be the users 'agent', acting on the user's behalf so that the user didn't have to understand the cryptic commands in order to retrieve information. Today, nearly everyone uses a web browser as their user agent. Different web browsers (Internet Explorer, Firefox, Opera, etc.) would therefore identify themselves with different user agent strings. When you visit a Web page, your browser sends the user- agent string to the server hosting the site that you are visiting. This string indicates which browser you are using, its version number, and details about your system, such as operating system and version. The Web server can use this information to provide content that is tailored for your specific browser. Because certain non-Microsoft sites add additional details to the user-agent string, it is important to understand the user-agent string. This article explains the user-agent string, lists the values from recent Internet Explorer versions, and documents registry keys that modify the user-agent string. (3) Message format Internetmessages consist of two major sections: Header Structured into fields such as summary, sender, receiver, and other information about the.fields Body The message itself as unstructured text; sometimes containing a signature block at the end. This is exactly the same as the body of a regular letter.signature block The header is separated from the body by a blank line. Message header Each message has exactly one header, which is structured into fields. Each field has a name and a value.headerfields Informally, each line of text in the header that begins with a printable character begins a separate field.printable character The field name starts in the first character of the line and ends before the separator character ":". The separator is then followed by the field value (the "body" of the field). The value is continued onto subsequent lines if those lines have a space or tab as their first character. Field names and values are restricted to 7-bit ASCII characters. Non-ASCII values may be represented using MIME encoded words.ASCIIencoded words Multipurpose Internet Mail Extensions (MIME) is an Internet standard that extends the format ofto support.Internet standard Text in character sets other than ASCIIcharacter setsASCII Non-text attachments Message bodies with multiple parts Header information in non-ASCII character sets Header fields The message header should include at least the following fields: From: Theaddress, and optionally the name of the author(s). In manyclients not changeable except through changing account settings. address To: Theaddress(es), and optionally name(s) of the message's recipient(s). Indicates primary recipients (multiple allowed), for secondary recipients see Cc: and Bcc: below. Subject: A brief summary of the topic of the message. Certain abbreviations are commonly used in the subject, including "RE:" and "FW:".Certain abbreviations"RE:" and "FW:" Date: The local time and date when the message was written. Like the From: field, manyclients fill this in automatically when sending. The recipient's client may then display the time in the format and time zone local to him/her. Message-ID: Also an automatically generated field; used to prevent multiple delivery and for reference. Message body: The main part of anmessage containing the actual, arbitrary data such as text or images. As opposed to the header, which contains control and meta-information. In the SMTP standard, the body is the fullmessage. The header here is only information servers need to deliver the message. Return the body of this message. BE WARNED that this returns you an object which may be encoded: use decoded() to get a body with usable data. With options, a new BODY is set for this message. This is not for normal use unless you understand the consequences: you change the message content without changing the message-ID. The BODY must be an (sub-)class of Mail::Message::Body. In this case, information from the specified body will be copied into the header. The body object will be encoded if needed, because messages written to file or transmitted shall not contain binary data. The converted body is returned.Mail::Message::Body When BODY is undef, the current message body will be dissected( affected, collected, complected, connected, directed) from the message. affected collectedcomplectedconnecteddirected All relation will be cut. The body is returned, and can be connected to a different message. Mail protocol:SMTP SMTP-simple mail transfer protocol Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail ( ) transmission across Internet Protocol (IP) networks.Internet standard electronic mailInternet Protocol SMTP is specified for outgoing mail transport and uses TCP port 25.TCPport While electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically only use SMTP for sending messages to a mail server for relaying.mail serversmail transfer agentsrelaying For receiving messages, client applications usually use either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) or a proprietary system (such as Microsoft Exchange or Lotus Notes/Domino) to access their mail box accounts on a mail server. Post Office ProtocolInternet Message Access Protocol Lotus NotesDomino SMTP is a text-based protocol, in which a mail sender communicates with a mail receiver by issuing command strings and supplying necessary data over a reliable ordered data stream channel, typically a Transmission Control Protocol (TCP) connection.text-basedTransmission Control Protocol An SMTP session consists of commands originated by an SMTP client (the initiating agent, sender, or transmitter) and corresponding responses from the SMTP server (the listening agent, or receiver) so that the session is opened, and session parameters are exchanged. A session may include zero or more SMTP transactions. An SMTP transaction consists of three command/reply sequences (see example below.) They are:clientagentserver MAIL command, to establish the return address, a.k.a. Return-Path, 5321.From, mfrom, or envelope sender. This is the address for bounce messages.bounce messages RCPT command, to establish a recipient of this message. This command can be issued multiple times, one for each recipient. These addresses are also part of the envelope. DATA to send the message text. This is the content of the message, as opposed to its envelope. It consists of a message header and a message body separated by an empty line. DATA is actually a group of commands, and the server replies twice: once to the DATA command proper, to acknowledge that it is ready to receive the text, and the second time after the end-of-data sequence, to either accept or reject the entire message. Besides the intermediate reply for DATA, each server's reply can be either positive (2xx reply codes) or negative. Negative replies can be permanent (5xx codes) or transient (4xx codes). A reject is a permanent failure by an SMTP server; in this case the SMTP client should send a bounce message. A drop is a positive response followed by message discard rather than delivery. The initiating host, the SMTP client, can be either an end-user'sclient, functionally identified as a mail user agent (MUA), or a relay server's mail transfer agent (MTA), that is an SMTP server acting as an SMTP client, in the relevant session, in order to relay mail. Fully-capable SMTP servers maintain queues of messages for retrying message transmissions that resulted in transient failures. clientmail user agentmail transfer agent A MUA knows the outgoing mail SMTP server from its configuration. An SMTP server acting as client, i.e. relaying, typically determines which SMTP server to connect to by looking up the MX (Mail exchange) DNS resource record for each recipient's domain name. Conformant MTAs (not all) fall back to a simple A record in case no MX record can be found. Relaying servers can also be configured to use a smart host.MXDNSdomain nameA record smart host An SMTP server acting as client initiates a TCP connection to the server on the "well-known port" designated for SMTP: port 25.TCPwell-known port POP3(POST OFFICE PROTOCOL) the Post Office Protocol (POP) is an application-layer Internet standard protocol used by localclients to retrievefrom a remote server over a TCP/IP connection.application-layerInternet standardprotocol clients serverTCP/IP POP and IMAP (Internet Message Access Protocol) are the two most prevalent Internet standard protocols forretrieval.IMAPInternet Virtually all modernclients and servers support both. The POP protocol has been developed through several versions, with version 3 (POP3) being the current standard. Like IMAP, POP3 is supported by most webmail services such as Hotmail, Gmail and Yahoo! Mail.serverswebmailHotmailGmailYahoo! Mail POP supports simple download-and-delete requirements for access to remote mailboxes (termed mail drop in the POP RFC's).RFC Although most POP clients have an option to leave mail on server after download,clients using POP generally connect, retrieve all messages, store them on the user's PC as new messages, delete them from the server, and then disconnect. Manyclients support POP as well as IMAP to retrieve messages; however, fewer Internet Service Providers (ISPs) support IMAP.Internet Service Providers A POP3 server listens on well-known port 110. Encrypted communication for POP3 is either requested after protocol initiation, using the STLS command, if supported, or by POP3S, which connects to the server using Transport Layer Security (TLS) or Secure Sockets Layer (SSL) on well-known TCP port 995 (e.g. Google Gmail).well-known portEncryptedTransport Layer Security Secure Sockets LayerGoogle Available messages to the client are fixed when a POP session opens the mail drop, and are identified by message-number local to that session or, optionally, by a unique identifier assigned to the message by the POP server. This unique identifier is permanent and unique to the mail drop and allows a client to access the same message in different POP sessions. Mail is retrieved and marked for deletion by message-number. When the client exits the session, the mail marked for deletion is removed from the mail drop. RFC- Request for Comment-a formal internet document 6.4 INTRANET An Intranet is an internal information system based on Internet technology, web services, TCP/IP and HTTP communication protocols, and HTML publishing. The Intranet is a technology that permits your organization to define itself as a whole entity, a group, a family, where everyone knows their roles, and everyone is working on the improvement and health of the organization. How do they work: By identifying and communicating missions, goals, processes, relationships, interactions, infrastructure, projects, schedules, budgets and culture on-line, in a single interface everyone uses and can add value back to. In a word, an Intranet represents your organizations intelligence. The purpose of this intelligence is to organize each individuals desktop with minimal cost, time and effort to be more productive, more cost efficient, more timely, and more competitive. Leveraging(to improve /advance) Intelligence: Intranets are not about putting technology and software together. Thats the easy part. In fact, youve probably got all the components in your organization already. Effectively building an Intranet is similar to building individual intelligence. It requires learning, applying the learning to practical decision making, acting on the intelligence with solid, clear tasks and responsibilities, modifying the learning for improved performance in the future, and making sure all of this is communicated all of the time to everyone. The Intranet is the WAN/LAN, client/server, PC, UNIX, Apple computer stuff that youve been using all along in your organization to do your work, improve efficiency, and communicate with others. The problem, of course, is that the machines, software, and communication systems have been proprietary. You couldn't have internal communication of all data and information without a team of programmers and new software for every new cut on the information. With an Intranet, you have access to all the information, applications, data, knowledge, processes, etc. available in the same window, or the same browser. No more conversion to different formats, waiting for programmers to code all the new systems together, or teams of consultants to sift through your processes. No more missed opportunities, giving up and not doing business with someone, because their technology was different. Organisational Focus: The Intranet is your opportunity to define your organization and display it for everyone to see. If everyone knows what the company stands for, what the companys strategic vision is, what the guiding company principles are, who the clients and partners are, then they can focus more clearly on what their own contributions are to the organization. A clear, single web page representing the values of the company is tantamount to success. Every organization can constantly refer to the central messages and develop their own supporting sites accordingly. Use the Web as an information, communications, and project- management tool across the organization. What the Intranet is Not The Internet is more of a marketing and customer support entity. The Intranet is your internal communications mechanism. 6.5 and 6.6 Read from tanenbum 1 st chapter.