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                <td bgcolor="#000000"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td class="rlohdr"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td valign="top" class="rlohdr">15.3</td><td width="100%" class="rlohdr">
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                            <td class="rlohdr">Network Applications</td>
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                            <td class="riohdr">DNS function</td>
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                                                                <td valign="middle"><img height="22" width="22" border="0" src="../../../../../images/ccna/common/inotes.gif"></td><td valign="middle"><span class="cstitle">Instructor Note</span></td>
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                                                                <td valign="top">&nbsp;</td><td valign="top"><span class="cstext">
                                                                        <p>The purpose of this target indicator is to relate email to a prior topic, DNS. DNS assists in the mail delivery process.</p>
                                                                        <p>Just as an e-mail message begins at a e-mail server (post office), so too much it end a an e-mail server (another post office). Then local delivery of the e-mail can be performed.</p>
                                                                        <p>Again, the common process of entering a password to retrieve your email is explained.</p>
                                                                        <p>E-mail is related to a prior topic, encapsulation, as it is a key theme of the entire semester and of the OSI model. Sending and receiving an e-mail involves encapsulation at all seven layers of the OSI model. This is an excellent opportunity to review the entire OSI model and what functions each layer performs.</p>
                                                                        <p>This target indicator describes in detail what a mail server does.</p>
                                                                        <p>This target indicator describes the file formats typically used with email. This is an excellent opportunity to review Layer 6 topics.</p>
                                                                        <p>This TI relates to CCNA Certification Exam Objectives #1 and #5.</p>
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<p>Whenever e-mail clients send letters, they request that a DNS server connected to the network translate the domain names into their associated IP addresses. If the DNS server is able to translate the names, it returns the IP addresses to the clients, thus enabling proper transport layer segmentation and encapsulation. If the DNS server cannot translate the names, the requests are passed on until the names can be translated.<img border="0" src="../../../../../CHAPID=knet-v214aCH47515/RLOID=knet-v214aRLO48009/RIOID=knet-v214aRIO124721/knet/v214adataimage1/1.gif" width="12" height="12"></p>
<p>The part of the e-mail address that contains the recipient's name becomes important at this point. The server extracts it from the e-mail message and checks to see if he is a member of its post office. If the recipient is a member, it stores the message in his mailbox until someone retrieves it. If the recipient is not a member, the post office generates an error message and sends the e-mail back to the sender.<img border="0" src="../../../../../CHAPID=knet-v214aCH47515/RLOID=knet-v214aRLO48009/RIOID=knet-v214aRIO124721/knet/v214adataimage4/4.gif" width="12" height="12"></p>
<p>The second part of the e-mailing process is the receiving process. E-mail message recipients must use the e-mail client software on their computers to establish requests to the e-mail post offices. When message recipients click the "Get Mail" or "Retrieve Mail" buttons on the e-mail client, they are usually prompted for a password. After they enter the password and click "OK", the e-mail software builds a request for the post office servers. It then extracts the post office addresses from the configuration data that was entered when their e-mail software was configured.<img border="0" src="../../../../../CHAPID=knet-v214aCH47515/RLOID=knet-v214aRLO48009/RIOID=knet-v214aRIO124721/knet/v214adataimage2/2.gif" width="12" height="12"> The process then uses another DNS search to find the IP addresses of the servers. Finally, the requests are segmented and sequenced by the transport layer.</p>
<p>Data packets travel through the rest of the OSI model layers (that is, network, data link, and physical) and are then transmitted across the Internet to the destination e-mail post office. At this post office the packets are reassembled, in the proper sequence, and are checked for any data transmission errors.<img border="0" src="../../../../../CHAPID=knet-v214aCH47515/RLOID=knet-v214aRLO48009/RIOID=knet-v214aRIO124721/knet/v214adataimage3/3.gif" width="12" height="12"></p>
<p>At the post office, requests are examined, and user names and passwords are verified. If everything is correct, the post office server transmits all e-mail messages to the user's computer.<img border="0" src="../../../../../CHAPID=knet-v214aCH47515/RLOID=knet-v214aRLO48009/RIOID=knet-v214aRIO124721/knet/v214adataimage4/4.gif" width="12" height="12"><img border="0" src="../../../../../CHAPID=knet-v214aCH47515/RLOID=knet-v214aRLO48009/RIOID=knet-v214aRIO124721/knet/v214adataimage5/5.gif" width="12" height="12"><img border="0" src="../../../../../CHAPID=knet-v214aCH47515/RLOID=knet-v214aRLO48009/RIOID=knet-v214aRIO124721/knet/v214adataimage6/6.gif" width="12" height="12"></p>
<p>After e-mail messages arrive at a computer, you may open them and read them. If you click on the "Reply", or the "Forward" button, to send a response to a messages, the whole process starts over again. E-mail messages are normally sent as ASCII text, but the attachments that are sometimes included with them can be audio, video, graphic, or many other types of data. To correctly send and receive attachments, the encoding schemes must be the same on both the sending and the receiving computer. The two most common formats for e-mail attachments are the Multipurpose Internet Mail Extension (MIME) and UUencode (a Unix utility program).</p>
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<a href="#" onClick="window.open('../../../../../CHAPID=null/RLOID=null/RIOID=knet-v214aRIOv2141010/knet/v214aRIOv2141010/iaactivity.html','LabActivity','width=742,height=390,toolbar=0,scrollbars=0,left=26,top=38');void(null);">Interactive Lab Activity</a>
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                                                                            <td valign="top">&nbsp;</td><td valign="top"><span class="cstext">
                                                                                    <p>DNS Lookup Sequence</p>
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