Chapter 15, Layer 7:  Application Layer

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

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

    Client Server

            Most applications that work in a networked enviroment are classified as client-server applications.

            These applications, such as FTP, web browsers, and e-mail, all have two components, which allow tham to function:
            1. The Client Side
            2. The Server Side

    Network Applications

            The World Wide Web (www), and the web browsers netscape and internet explorer are probably the most
            commonly used network applications.

    Network Redirector

            Redirector is a protocol that works with computer operating systems and network clients instead of specific
            application programs.

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

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

    Two Types of Communication

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

            All communication activity falls into one of these two categories.

    Make a Connection
    Break a Connection

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

            When downloading a webpage, the connection to the remote serverwill be maintained only long enough to process
            the transaction.

    Client Determines Termination

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

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

    IP Addresses

            The internet is built on a hierarchial dressing scheme.  This allows for routing that is based on classes of addresses,
            as opposed to individual addresses.


            A domain is a group of computers that are associated by their geographical location or their business type.

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

            In the URL, the "" part defines 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 addresss.


            E-mail cliant applications, such as Eudora, Outlook Express, and Netscape Mail, work with the POP3 protocol.

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


            Terminal Emulation (Telnet) software provides the abiltiy to remortely access another computer.

            The telnet application works mainly on the application, resentation, and session layers of the OSI model.