Chabot College ELEC 99.05 Application Layer Puzzles CISCO NETWORKING ACADEMY Puzzle • When I run a network application, I see results on my screen. • Where are those events happening? CISCO NETWORKING ACADEMY Application Layer • Host relationships: – Client • typically a user PC • the source of a transaction – Server • a shared host on the network • the destination of the transaction • Network applications usually are “client-server” CISCO NETWORKING ACADEMY Application Layer • Connection types – Make-Break – Session CISCO NETWORKING ACADEMY Make-Break Connections • Typically involve a single file transfer. • Examples – HTTP • HyperText Transfer Protocol • web file • TCP port 80 – SMTP • Simple Mail Transfer Protocol • E-mail file • TCP port 25 CISCO NETWORKING ACADEMY Session Connections • Maintained until “session” is closed. • Examples – telnet • terminal emulation • passes keystrokes from client to server • TCP port 23 – FTP • File Tranfer Protocol • manages transfer of many files • TCP port 21 CISCO NETWORKING ACADEMY telnet Examples • telnet://library.uark.edu • telnet://locis.loc.gov CISCO NETWORKING ACADEMY Running telnet (DOS) • At the DOS Prompt, type >telnet hostname or >telnet Ipaddress • Try it! Demo Telnet to library.uark.edu • Be sure to choose “Q” to logoff when done. CISCO NETWORKING ACADEMY Running telnet (Windows) • Start | Programs | Accessories | Communications | Hyperterminal • Double-click Hypertrm.exe • Enter a name for this connection • At the bottom of the next dialog box, for “Connect Using”, select TCP/IP (winsock) • Add the destination host name or IP address • (Notice the port number in this dialog) • Try it! Demo Telnet to library.uark.edu • Be sure to choose “Q” to logoff when done. • When asked if you wish to save the session, say “No” CISCO NETWORKING ACADEMY Running telnet (browser) • Browsers use helper applications to provide certain functions. • By default, Internet Explorer launches Hyperterm when it encounters a URL that specifies the telnet protocol • Try it! Demo • Point your browser at telnet://library.uark.edu • Be sure to choose “Q” to logoff when done. • When asked if you wish to save the session, say “No” CISCO NETWORKING ACADEMY telnet • In Semester 2, you will use telnet extensively to connect your PC to routers. CISCO NETWORKING ACADEMY DNS Puzzler • In the following URL, what does “WWW” refer to? • http://www.clpccd.cc.ca.us • Answer: It is the hostname (computer name) portion of a “fully qualified domain name” It does not specify the protocol HTTP (web)! This computer just happens to be named “www”. CISCO NETWORKING ACADEMY Fully Qualified Domain Name www.clpccd.cc.ca.us Host name CISCO NETWORKING ACADEMY Domain name Top-Level Domain name Let’s try it! • Point your browser at: • http://209.129.156.3 • http://www.clpccd.cc.ca.us • http://www • http://porter.clpccd.cc.ca.us • http://porter CISCO NETWORKING ACADEMY Let’s try it! • For the host at 209.129.156.3, Chabot’s DNS servers list • porter as the host name • www as a name alias (a canonical name) • We will study the structure of DNS tables in Semester 2. CISCO NETWORKING ACADEMY DNS Puzzler • Why is Chabot-Las Positas assigned a domain name of clpccd.cc.ca.us ? • Why aren’t we clpccd.edu ? • Answer: RFC 1480 • A recent change permits community colleges to move to the .edu top-level domain. CISCO NETWORKING ACADEMY Browser URL • Universal Resource Locator • Example: http://www.cisco.com protocol hostname domain top-level domain • Browser uses the text before :// to select protocol and port number. CISCO NETWORKING ACADEMY Let’s try it! • Point your browser at: • telnet://library.uark.edu • http://library.uark.edu CISCO NETWORKING ACADEMY Browser URL • Universal Resource Locator • Example: http://www.cisco.com • If protocol:// is omitted, then the protocol is not specified. • In that case, the browser tries to “guess” the protocol from the hostname: – hostname www defaults to http (port 80) – hostname ftp defaults to ftp (port 21) CISCO NETWORKING ACADEMY HTTP Puzzler • When you view a web page, where is it? • Answer: On your PC! • Let’s look at the contents of C:\windows\temporary internet files demo • Note that one web page may be made up of many files of various types. • Note that one web page may contain files loaded from several servers! CISCO NETWORKING ACADEMY E-mail Puzzler • When you view your E-mail, where is it stored? • Answer It depends on the protocol! – Post Office Protocol (POP) – Web mail (e.g. yahoo mail) CISCO NETWORKING ACADEMY E-mail Clients • POP Clients – – – – E-mail is transferred from the mail server to your PC Port 110 Client examples” Eudora, Outlook Express, Netscape Most ISPs offer POP E-mail • Web mail clients – E-mail remains on the server for viewing by any browser in the world. – Port 80 – Browser is the client. – Offered by Yahoo, Hotmail, GroupWise Web Access CISCO NETWORKING ACADEMY POP E-mail SMTP Server/ POP Server POP (110) SMTP Server/ POP Server SMTP (25) POP (110) E-mail passes through the post office server but does not remain there. E-mail is delivered to client PC. Once delivered, it can only be accessed at that PC. CISCO NETWORKING ACADEMY Web E-mail SMTP Server/ HTTP Server HTTP (80) SMTP Server/ HTTP Server SMTP (25) HTTP (80) E-mail is delivered to the Web mail server and remains there. Web E-mail can be viewed from any web browser. CISCO NETWORKING ACADEMY