Layer 7 The Application Layer Chapter Review

Layer 7 The Application Layer zThe application layer is the OSI layer closest to the end system. This determines whether sufficient resources exist for communication between systems. zThe application layer does not provide support for any other OSI layer.

Client - Server zMost applications that work in a networked environment are classified as client-server applications. zThese applications, such as FTP, web browsers, and , all have two components, which allow them to function: 1. The client side 2. The server side

Network Applications The World Wide Web (WWW), and the web browsers Netscape Navigator, and Internet Explorer, are probably the most commonly used network applications.

Network Redirector zRedirector is a protocol that works with computer operating systems and network clients instead of specific application programs. zIf a client wants to save a file from a word processor to a network server, the redirector enables the word processing application to become a network client. zA network redirector enables data to be sent to a device that is not directly connected to a computer.

Two Types of Communication zMake a connection - break a connection and termination determined by client. zAll communication activity falls into one of these two categories.

Make a Connection Break a Connection zThe common cycle; make a connection - break a connection, between client and server, underlies all Web Page requests. zWhen downloading a web page, the connection to the remote server will be maintained only long enough to process the transaction.

Client Determines Termination zThis is illustrated by the Telnet and FTP examples, which establish a connection to the server, and maintains that connection until all processing has been performed. z The client computer terminates the connection when the user determines that he/she has finished.

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

Domains zA domain is a group of computers that are associated by their geographical location or their business type. zA domain name is a string of characters and/or numbers that represents the numeric address of an Internet site. zIn the URL the “cisco.com” part identifies the domain.

DNS - Domain Name Server A DNS server manages domain names and responds to requests from clients to translate a domain name into the associated IP address.

z client applications, such as Eudora, Outlook Express, and Netscape Mail, work with the POP3 protocol. zFor security, when message recipients check their they are often prompted for a password. The password can also be saved in many programs. zAn address consists of the recipient’s username and postoffice address.

Telnet zTerminal emulation (Telnet) software provides the ability to remotely access another computer. zThe Telnet application works mainly at the application, presentation and session layers of the OSI model.

FTP zFile transfer protocol (FTP) is designed to download files or upload files on the Internet. zAn FTP session is established the same way in which a Telnet session is established. Just like Telnet, the FTP session is maintained until the client terminates it, or there is some sort of communication error.

Web Pages zWeb pages are created with a format language called Hypertext Markup Language (HTML). zHyperlinks make the World Wide Web easy to navigate. A hyperlink is an object on a Web page that, when clicked, transfers you to a new Web page.

Conclusion zThis concludes our review of chapter 15, The Application Layer. zCongratulations on getting through all 15 chapters of Cisco Semester 1! zNow you’re prepared to take the chapter 15 online exam.