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<html> <head> <meta content="en-us" http-equiv="Content-Language"> <meta content="text/html; charset=UTF-8" http-equiv="Content-Type"> <link type="text/css" rel="stylesheet" href="../../../../../css/ccna.css"> <script src="../../../../../js/framesetHelper.js" language="JavaScript"></script><script language="JavaScript"> window.scrollTo(0,0); function newPage(pageID) { alert("COMMING SOON!!\nThis feature is currently in development."); } function launchMoreInfo(oid) { dash = oid.indexOf('-'); sub1 = oid.substring(0,dash); sub2 = oid.substring(dash+1); // this is an URL base on the // system that generated the static // download of the curriculum. httpref = "http://nomar.sdf.cisco.com/servlet/org.cli.delivery.rendering.servlet.MIServlet/Static=true,LMSID=DTUI,Engine=dynamic,Theme=cnamstheme,Style=ccna,Language=en,Version=2,RootID=knet-v214aCV47051/CHAPID=null/RLOID=null/RIOID=null/"; newref = httpref + sub1 +"/"+ sub2 +"/moreinfoframeset.html"; //alert("new newref :\n" + newref); window.open(newref,'MoreInfo','toolbar=no,status=yes,menubar=no,location=no,scrollbars=yes,height=442,width=756,left=20,top=20,resizable=yes') } function launchContentResource(contentID) { top.currentSupport = 0; 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The purpose is to illustrate that the way TCP provides software services to upper layers is through these numbers -- they are a menu of services. Students should know the port numbers for ftp, Telnet, smtp, dns, tftp, and snmp.</p> <p>This TI relates to CCNA Certification Exam Objectives #1, #6, and #35.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>Both TCP and UDP use <i>port (or socket)</i> numbers to pass information to the upper layers. Port numbers are used to keep track of different conversations that cross the network at the same time. Application software developers have agreed to use the well-known port numbers that are defined in RFC1700. Any conversation bound for the FTP application uses the standard port number 21.<img border="0" src="../../../../../CHAPID=knet-v214aCH47512/RLOID=knet-v214aRLO47841/RIOID=knet-v214aRIO124689/knet/v214adataimage1/1.gif" width="12" height="12"> Conversations that do not involve applications with well-known port numbers are assigned port numbers that have been randomly selected from within a specific range. These port numbers are used as source and destination addresses in the TCP segment.<img border="0" src="../../../../../CHAPID=knet-v214aCH47512/RLOID=knet-v214aRLO47841/RIOID=knet-v214aRIO124689/knet/v214adataimage4/4.gif" width="12" height="12"></p><p>Some ports are reserved in both TCP <img border="0" src="../../../../../CHAPID=knet-v214aCH47512/RLOID=knet-v214aRLO47841/RIOID=knet-v214aRIO124689/knet/v214adataimage2/2.gif" width="12" height="12"> and UDP <img border="0" src="../../../../../CHAPID=knet-v214aCH47512/RLOID=knet-v214aRLO47841/RIOID=knet-v214aRIO124689/knet/v214adataimage3/3.gif" width="12" height="12">, although applications might not be written to support them. Port numbers have the following assigned ranges:</p><ul type="disc"><li>Numbers below 255 - for public applications</li><li>Numbers from 255-1023 - assigned to companies for marketable applications</li><li>Numbers above 1023 - are unregulated</li></ul><p>End systems use port numbers to select proper applications. Originating source port numbers are dynamically assigned by the source host. Usually, it is a number larger than 1023.</p></td> </tr> </table> </td> </tr></table> <p> </p> <table width="100%" cellspacing="0" cellpadding="2" border="0" valign="bottom"> <tr> <td></td> </tr> </table> </body></html>⌨️ 快捷键说明
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