Fundamental Information Technology Notes

UNIT-4 client/server Client/server describes the relationship between two computer programs in which one program, the client, makes a service request from another program, the server, which fulfills the request. Although the client/server idea can be used by programs within a single computer, it is a more important idea in a network. In a network, the client/server model provides a convenient way to interconnect programs that are distributed efficiently across different locations. Computer transactions using the client/server model are very common. For example, to check your bank account from your computer, a client program in your computer forwards your request to a server program at the bank. That program may in turn forward the request to its own client program that sends a request to a database server at another bank computer to retrieve your account balance. The balance is returned back to the bank data client, which in turn serves it back to the client in your personal computer, which displays the information for you. The client/server model has become one of the central ideas of network computing. Most business applications being written today use the client/server model. So does the Internet's main program, TCP/IP. In marketing, the term has been used to distinguish distributed computing by smaller dispersed computers from the "monolithic" centralized computing of mainframe computers. But this distinction has largely disappeared as mainframes and their applications have also turned to the client/server model and become part of network computing. In the usual client/server model, one server, sometimes called a daemon, is activated and awaits client requests. Typically, multiple client programs share the services of a common server program. Both client programs and server programs are often part of a larger program or application. Relative to the Internet, your Web browser is a client program that requests services (the sending of Web pages or files) from a Web server (which technically is called a Hypertext Transport Protocol or HTTP server) in another computer somewhere on the Internet. Similarly, your computer with TCP/IP installed allows you to make client requests for files from File Transfer Protocol (FTP) servers in other computers on the Internet. Other program relationship models included master/slave, with one program being in charge of all other programs, and peer-to-peer, with either of two programs able to initiate a transaction.

intranet An intranet is a private network that is contained within an enterprise. It may consist of many interlinked local area networks and also use leased lines in the wide area network. Typically, an intranet includes connections through one or more gateway computers to the outside Internet. The main purpose of an intranet is to share company information and computing resources among employees. An intranet can also be used to facilitate working in groups and for teleconferences. An intranet uses TCP/IP, HTTP, and other Internet protocols and in general looks like a private version of the Internet. With tunneling, companies can send private messages through the public network, using the public network with special encryption/decryption and other security safeguards to connect one part of their intranet to another. Typically, larger enterprises allow users within their intranet to access the public Internet through firewall servers that have the ability to screen messages in both directions so that company security is maintained. When part of an intranet is made accessible to customers partners, suppliers, or others outside the company, that part becomes part of an extranet.

extranet An extranet is a private network that uses Internet technology and the public telecommunication system to securely share part of a business's information or operations with suppliers, vendors, partners, customers, or other businesses. An extranet can be viewed as part of a company's intranet that is extended to users outside the company. It has also been described as a "state of mind" in which the Internet is perceived as a way to do business with other companies as well as to sell products to customers. An extranet requires security and privacy. These can include firewall server management, the issuance and use of digital certificates or similar means of user authentication, encryption of messages, and the use of virtual private networks (VPNs) that tunnel through the public network. Companies can use an extranet to:      

Exchange large volumes of data using Electronic Data Interchange (EDI) Share product catalogs exclusively with wholesalers or those "in the trade" Collaborate with other companies on joint development efforts Jointly develop and use training programs with other companies Provide or access services provided by one company to a group of other companies, such as an online banking application managed by one company on behalf of affiliated banks Share news of common interest exclusively with partner companies

domain name A domain name locates an organization or other entity on the Internet. For example, the domain name www.totalbaseball.com locates an Internet address for "totalbaseball.com" at Internet point 199.0.0.2 and a particular host server named "www." The "com" part of the domain name reflects the purpose of the organization or entity (in this example, "commercial") and is called the top-level domain name. The "totalbaseball" part of the domain name defines the organization or entity and together with the top-level is called the second-level domain name. The second-level domain name maps to and can be thought of as the "readable" version of the Internet address. A third level can be defined to identify a particular host server at the Internet address. In our example, "www" is the name of the server that handles Internet requests. (A second server might be called "www2.") A third level of domain name is not required. For example, the fullyqualified domain name could have been "totalbaseball.com" and the server assumed. Subdomain levels can be used. For example, you could have "www.nyyankees.totalbaseball.com." Together, "www.totalbaseball.com" constitutes a fullyqualified domain name. Second-level domain names must be unique on the Internet and registered with one of the ICANN-accredited registrars for the COM, NET, and ORG top-level domains. Where appropriate, a top-level domain name can be geographic. (Currently, most non-U.S. domain names use a top-level domain name based on the country the server is in.) To register a U. S. geographic domain name or a domain name under a country code, see an appropriate registrar. On the Web, the domain name is that part of the Uniform Resource Locator(URL) that tells a domain name server using the domain name system (DNS) whether and where to forward a request for a Web page. The domain name is mapped to an IP address (which represents a physical point on the Internet). More than one domain name can be mapped to the same Internet address. This allows multiple individuals, businesses, and organizations to have separate Internet identities while sharing the same Internet server. To see the IP address for a domain name, (Ping). It may be worth noting that the domain name system contains an even higher level of domain than the top-level domain. The highest level is the root domain, which would be represented by a single dot (just as in many hierarchical file systems, a root directory is represented by a "/" ) if it were ever used. If the dot for the root domain were shown in the URL, it would be to the right of the top-level domain name. However, the dot is assumed to be there, but never shown.

IP address (Internet Protocol Address) This definition is based on Internet Protocol Version 4. See Internet Protocol Version 6 (IPv6) for a description of the newer 128-bit IP address. Note that the system of IP address classes described here, while forming the basis for IP address assignment, is generally bypassed today by use of Classless Inter-Domain Routing (CIDR) addressing.

In the most widely installed level of the Internet Protocol (IP) today, an IP address is a 32-bit number that identifies each sender or receiver of information that is sent in packets across the Internet. When you request an HTML page or send e-mail, the Internet Protocol part of TCP/IP includes your IP address in the message (actually, in each of the packets if more than one is required) and sends it to the IP address that is obtained by looking up the domain name in the Uniform Resource Locator you requested or in the e-mail address you're sending a note to. At the other end, the recipient can see the IP address of the Web page requestor or the e-mail sender and can respond by sending another message using the IP address it received. An IP address has two parts: the identifier of a particular network on the Internet and an identifier of the particular device (which can be a server or a workstation) within that network. On the Internet itself - that is, between the router that move packets from one point to another along the route - only the network part of the address is looked at. The Network Part of the IP Address

The Internet is really the interconnection of many individual networks (it's sometimes referred to as an internetwork). So the Internet Protocol (IP) is basically the set of rules for one network communicating with any other (or occasionally, for broadcast messages, all other networks). Each network must know its own address on the Internet and that of any other networks with which it communicates. To be part of the Internet, an organization needs an Internet network number, which it can request from the Network Information Center (NIC). This unique network number is included in any packet sent out of the network onto the Internet. The Local or Host Part of the IP Address

In addition to the network address or number, information is needed about which specific machine or host in a network is sending or receiving a message. So the IP address needs both the unique network number and a host number (which is unique within the network). (The host number is sometimes called a local or machine address.) Part of the local address can identify a subnetwork or subnet address, which makes it easier for a network that is divided into several physical subnetworks (for examples, several different local area networks or ) to handle many devices. IP Address Classes and Their Formats

Since networks vary in size, there are four different address formats or classes to consider when applying to NIC for a network number:

   

Class A addresses are for large networks with many devices. Class B addresses are for medium-sized networks. Class C addresses are for small networks (fewer than 256 devices). Class D addresses are multicast addresses.

The first few bits of each IP address indicate which of the address class formats it is using. The address structures look like this: Class A 0

Network (7 bits)

Local address (24 bits)

Class B 10

Network (14 bits)

Local address (16 bits)

Class C 110

Network (21 bits)

Local address (8 bits)

Class D 1110

Multicast address (28 bits)

The IP address is usually expressed as four decimal numbers, each representing eight bits, separated by periods. This is sometimes known as the dot address and, more technically, as dotted quad notation. For Class A IP addresses, the numbers would represent "network.local.local.local"; for a Class C IP address, they would represent "network.network.network.local". The number version of the IP address can (and usually is) represented by a name or series of names called the domain name. The Internet's explosive growth makes it likely that, without some new architecture, the number of possible network addresses using the scheme above would soon be used up (at least, for Class C network addresses). However, a new IP version, IPv6, expands the size of the IP address to 128 bits, which will accommodate a large growth in the number of network addresses. For hosts still using IPv4, the use of subnets in the host or local part of the IP address will help reduce new applications for network numbers. In addition, most sites on today's mostly IPv4 Internet have gotten around the Class C network address limitation by using the Classless Inter-Domain Routing (CIDR) scheme for address notation.

Relationship of the IP Address to the Physical Address

The machine or physical address used within an organization's local area networks may be different than the Internet's IP address. The most typical example is the 48-bit Ethernet address. TCP/IP includes a facility called the Address Resolution Protocol (ARP) that lets the administrator create a table that maps IP addresses to physical addresses. The table is known as the ARP cache. Static versus Dynamic IP Addresses

The discussion above assumes that IP addresses are assigned on a static basis. In fact, many IP addresses are assigned dynamically from a pool. Many corporate networks and online services economize on the number of IP addresses they use by sharing a pool of IP addresses among a large number of users. If you're an America Online user, for example, your IP address will vary from one logon session to the next because AOL is assigning it to you from a pool that is much smaller than AOL's base of subscribers.

URL (Uniform Resource Locator) A URL (Uniform Resource Locator, previously Universal Resource Locator) - usually pronounced by sounding out each letter but, in some quarters, pronounced "Earl" - is the unique address for a file that is accessible on the Internet. A common way to get to a Web site is to enter the URL of its home page file in your Web browser's address line. However, any file within that Web site can also be specified with a URL. Such a file might be any Web (HTML) page other than the home page, an image file, or a program such as a common gateway interface application or Java applet. The URL contains the name of the protocol to be used to access the file resource, a domain name that identifies a specific computer on the Internet, and a pathname, a hierarchical description that specifies the location of a file in that computer. On the Web (which uses the Hypertext Transfer Protocol, or HTTP), an example of a URL is:

http://www.ietf.org/rfc/rfc2396.txt

which specifies the use of a HTTP (Web browser) application, a unique computer named www.ietf.org, and the location of a text file or page to be accessed on that computer whose pathname is /rfc/rfc2396.txt. A URL for a particular image on a Web site might look like this: http://searchnetworking.techtarget.com/WhatIs/images/coaxla.gif

A URL for a file meant to be downloaded using the File Transfer Protocol (FTP) would require that the "ftp" protocol be specified like this hypothetical URL: ftp://www.somecompany.com/whitepapers/widgets.ps

A URL is a type of URI (Uniform Resource Identifier, formerly called Universal Resource Identifier.)

chat On the Internet, chatting is talking to other people who are using the Internet at the same time you are. Usually, this "talking" is the exchange of typed-in messages requiring one site as the repository for the messages (or "chat site") and a group of users who take part from anywhere on the Internet. In some cases, a private chat can be arranged between two parties who meet initially in a group chat. Chats can be ongoing or scheduled for a particular time and duration. Most chats are focused on a particular topic of interest and some involve guest experts or famous people who "talk" to anyone joining the chat. (Transcripts of a chat can be archived for later reference.) Chats are conducted on online services (especially America Online), by bulletin board services, and by Web sites. Several Web sites, notably Talk City, exist solely for the purpose of conducting chats. Some chat sites such as Worlds Chat allow participants to assume the role or appearance of an avatar in a simulated or virtual reality environment. Talk City and many other chat sites use a protocol called Internet Relay Chat. A chat can also be conducted using sound or sound and video, assuming you have the bandwidth access and the appropriate programming.

Instant messaging (IM) is a form of communication over the Internet, that offers quick transmission of text-based messages from sender to receiver. In push mode between two or more people using personal computers or other devices, along with shared clients, instant messaging basically offers realtime direct written language-based online chat. The user's text is conveyed over a network, such as the Internet. It may address point-to-point communications as well as multicast communications from one sender to many receivers. More advanced instant messaging allows enhanced modes of communication, such as live voice or video calling, video chat and inclusion of hyperlinks to media. 

Definition Instant messaging falls under the term online chat, since it is also text-based, bi-directionally exchanged, and happens in real-time. IM is distinct from chat in that IM is based on clients that facilitate connections between specified known users (often using a contact list, buddy list, or friend list). Online 'chat' includes web-based applications that allow communication between (often directly addressed, but anonymous) users in a multi-user environment.

Overview Instant messaging is a set of communication technologies used for text-based communication between two or more participants over the Internet or other types of networks. IM–chat happens in real-time. Of importance is that online chat and instant messaging differ from other technologies such as email due to the perceived quasi-synchronicity of the communications by the users. Some systems permit messages to be sent to users not then 'logged on' (offline messages), thus removing some differences between IM and email (often done by sending the message to the associated email account). IM allows effective and efficient communication, allowing immediate receipt of acknowledgment or reply. However IM is basically not necessarily supported by transaction control. In many cases, instant messaging includes added features which can make it even more popular. For example, users may see each other via webcams, or talk directly for free over the Internet using a microphone and headphones or loudspeakers. Many client programs allow file transfers, although they are usually limited in the permissible file-size. It is usually possible to save a text conversation for later reference. Instant messages are often logged in a local message history, making it similar to the persistent nature of emails.