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Layer 7Layer 7The Application LayerThe Application Layer

Chapter ReviewChapter Review

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Layer 7Layer 7The Application LayerThe Application Layer

The application layer is the OSI layer The application layer is the OSI layer closest to the end system. This determines closest to the end system. This determines whether sufficient resources exist for whether sufficient resources exist for communication between systems.communication between systems.

The application layer does not provide The application layer does not provide support for any other OSI layer. support for any other OSI layer.

Institute of Technology Sligo - Dept of Computing

Client - ServerClient - Server Most applications that work in a networked Most applications that work in a networked

environment are classified as client-server environment are classified as client-server applications. applications.

These applications, such as FTP, web These applications, such as FTP, web browsers, and e-mail, all have two browsers, and e-mail, all have two components, which allow them to function: components, which allow them to function: 1. The client side1. The client side

2. The server side2. The server side

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Network ApplicationsNetwork Applications

The World Wide Web (WWW), and the web The World Wide Web (WWW), and the web browsers Netscape Navigator, and browsers Netscape Navigator, and

Internet Explorer, are probably the most Internet Explorer, are probably the most commonly used network applications. commonly used network applications.

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Network RedirectorNetwork Redirector Redirector is a protocol that works with Redirector is a protocol that works with

computer operating systems and network clients computer operating systems and network clients instead of specific application programs. instead of specific application programs.

If a client wants to save a file from a word If a client wants to save a file from a word processor to a network server, the redirector processor to a network server, the redirector enables the word processing application to enables the word processing application to become a network client. become a network client.

A network redirector enables data to be sent to A network redirector enables data to be sent to a device that is not directly connected to a a device that is not directly connected to a computer.computer.

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Two Types of CommunicationTwo Types of Communication

Make a connection - break a connection Make a connection - break a connection and termination determined by client.and termination determined by client.

All communication activity falls into one of All communication activity falls into one of these two categories. these two categories.

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Make a ConnectionMake a ConnectionBreak a ConnectionBreak a Connection

The common cycle; make a connection - The common cycle; make a connection - break a connection, between client and break a connection, between client and server, underlies all Web Page requests.server, underlies all Web Page requests.

When downloading a web page, the When downloading a web page, the connection to the remote server will be connection to the remote server will be maintained only long enough to process maintained only long enough to process the transaction.the transaction.

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Client Determines TerminationClient Determines Termination

This is illustrated by the Telnet and FTP This is illustrated by the Telnet and FTP examples, which establish a connection to examples, which establish a connection to the server, and maintains that connection the server, and maintains that connection until all processing has been performed.until all processing has been performed.

The client computer terminates the The client computer terminates the connection when the user determines that connection when the user determines that he/she has finished.he/she has finished.

Institute of Technology Sligo - Dept of Computing

IP AddressesIP Addresses

the Internet is built on a hierarchical the Internet is built on a hierarchical addressing scheme. This allows for routing addressing scheme. This allows for routing that is based on classes of addresses, as that is based on classes of addresses, as

opposed to individual addresses.opposed to individual addresses.

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DomainsDomains A domain is a group of computers that are A domain is a group of computers that are

associated by their geographical location associated by their geographical location or their business type. or their business type.

A domain name is a string of characters A domain name is a string of characters and/or numbers that represents the and/or numbers that represents the numeric address of an Internet site. numeric address of an Internet site.

In the URL http://www.cisco.com, the In the URL http://www.cisco.com, the “cisco.com” part identifies the domain.“cisco.com” part identifies the domain.

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DNS - Domain Name ServerDNS - Domain Name Server

A DNS server manages domain names and A DNS server manages domain names and responds to requests from clients to responds to requests from clients to

translate a domain name into the translate a domain name into the associated IP address.associated IP address.

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E-mailE-mail E-mail client applications, such as Eudora, E-mail client applications, such as Eudora,

Outlook Express, and Netscape Mail, work with Outlook Express, and Netscape Mail, work with the POP3 protocol.the POP3 protocol.

For security, when message recipients check For security, when message recipients check their e-mail they are often prompted for a their e-mail they are often prompted for a password. The password can also be saved in password. The password can also be saved in many e-mail programs.many e-mail programs.

An e-mail address consists of the recipient’s An e-mail address consists of the recipient’s username and postoffice address.username and postoffice address.

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TelnetTelnet

Terminal emulation (Telnet) software Terminal emulation (Telnet) software provides the ability to remotely access provides the ability to remotely access another computer.another computer.

The Telnet application works mainly at the The Telnet application works mainly at the application, presentation and session application, presentation and session layers of the OSI model.layers of the OSI model.

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FTPFTP File transfer protocol (FTP) is designed to File transfer protocol (FTP) is designed to

download files or upload files on the Internet.download files or upload files on the Internet. An FTP session is established the same way in An FTP session is established the same way in

which a Telnet session is established. Just like which a Telnet session is established. Just like Telnet, the FTP session is maintained until the Telnet, the FTP session is maintained until the client terminates it, or there is some sort of client terminates it, or there is some sort of communication error. communication error.

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Web PagesWeb Pages

Web pages are created with a format Web pages are created with a format language called Hypertext Markup language called Hypertext Markup Language (HTML).Language (HTML).

Hyperlinks make the World Wide Web Hyperlinks make the World Wide Web easy to navigate. A hyperlink is an object easy to navigate. A hyperlink is an object on a Web page that, when clicked, on a Web page that, when clicked, transfers you to a new Web page. transfers you to a new Web page.

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ConclusionConclusion

This concludes our review of chapter 15, This concludes our review of chapter 15, The Application Layer. The Application Layer.

Congratulations on getting through all 15 Congratulations on getting through all 15 chapters of Cisco Semester 1!chapters of Cisco Semester 1!

Now you’re prepared to take the chapter Now you’re prepared to take the chapter 15 online exam.15 online exam.