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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; dash = contentID.indexOf('-'); sub1 = contentID.substring(0,dash); sub2 = contentID.substring(dash+1); numbackParams = "CHAPID=" + "knet-v214aCH47511" + "/RLOID=" + "knet-v214aRLO47824" + "/RIOID=" + "knet-v214aRIO123690"; ContentRef = "../../../../../"+ numbackParams + "/"+ sub1 +"/"+ sub2 +"/"; eval('parent.frames.content_frame.location.href="'+ContentRef+'content.html"'); eval('parent.frames.media_frame.location.href="'+ContentRef+'media.html"'); eval('parent.frames.navigation_frame.location.href="'+ContentRef+'contentresourcenav.html"'); top.currentSupport++; top.firstSupportPage = contentID; top.prevPage = contentID; top.contentResourceID = contentID; } function launchReference(rioID, rloID, chapterID) { contentID = "knet-v214aCV47051"; dash = contentID.indexOf('-'); sub1 = contentID.substring(0,dash); sub2 = contentID.substring(dash+1); numbackParams = "CHAPID=" + chapterID + "/RLOID=" + rloID + "/RIOID=" + rioID; ContentRef = "../../../../../"+ numbackParams + "/"+ sub1 +"/"+ sub2 +"/"; eval('top.frames.main_frame.location.href="'+ContentRef+'referencecontroller_crf.html"'); chapter = "knet-v214aCH47511"; ch_dash = chapter.indexOf('-'); ch_sub1 = chapter.substring(0,ch_dash); ch_sub2 = chapter.substring(ch_dash+1); params = "CHAPID=" + chapter + "/RLOID=" + "knet-v214aRLO47824" + "/RIOID=" + "knet-v214aRIO123690"; returnRef = "../../../../../"+ params + "/"+ ch_sub1 +"/"+ ch_sub2 +"/pageframeset.html"; top.prevPage = returnRef; } function launchFile(filepath) { newWindow = window.open(filepath,"","top=70,left=120,width="+(screen.width*.50)+",height="+(screen.height*.50)+",resizable=yes,scrollbars=yes,toolbar=yes,menubar=yes,status=yes"); /* if (is.ie && filepath.lastIndexOf(".pdf") != -1) { pageTimer = setInterval("winLoadCheck()", 1000); } */ } function winLoadCheck() { if (newWindow.document.readyState == 'complete') { clearInterval(pageTimer); newWindow.location.reload(); } } // BrowserCheck Object // provides most commonly needed browser checking variables // 19990326 // Copyright (C) 1999 Dan Steinman // Distributed under the terms of the GNU Library General Public License // Available at http://www.dansteinman.com/dynapi/ function BrowserCheck() { var b = navigator.appName if (b=="Netscape") this.b = "ns" else if (b=="Microsoft Internet Explorer") this.b = "ie" else this.b = b this.v = parseInt(navigator.appVersion) this.ns = (this.b=="ns" && this.v>=4) this.ns4 = (this.b=="ns" && this.v==4) this.ns5 = (this.b=="ns" && this.v==5) this.ie = (this.b=="ie" && this.v>=4) this.ie4 = (navigator.userAgent.indexOf('MSIE 4')>0) this.ie5 = (navigator.appVersion.indexOf('MSIE 5.0')>0) this.ie55 = (navigator.appVersion.indexOf('MSIE 5.5')>0) if (this.ie5) this.v = 5 this.min = (this.ns||this.ie) } // automatically create the "is" object is = new BrowserCheck()</script> </head> <body onLoad="window.focus();" alink="#000000" vlink="#000000" link="#000000" bgcolor="#E7E7E7" background="../../../../../images/ccna/common/bg.gif"> <table width="100%" cellspacing="0" cellpadding="2" border="0"><tr> <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">11.1</td><td width="100%" class="rlohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="rlohdr">Layer 3 Devices</td> </tr> </table> </td> </tr> <tr> <td bgcolor="#000000"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td class="riohdr"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td valign="top" class="riohdr">11.1.3</td><td width="100%" class="riohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="riohdr">Unique network numbers</td> </tr> </table> </td> </tr> <tr> <td colspan="4"> <table width="90%" cellspacing="0" cellpadding="0" border="0"> <tr> <td class="smtext"> <p> </p><table cellpadding="0" cellspacing="0" width="100%" border="0" xmlns:java="http://xml.apache.org/xslt/java"> <tr> <td width="100%"><img height="10" width="1" border="0" src="../../../../../images/ccna/common/transdot.gif"></td> </tr> <tr> <td width="100%"> <table bgcolor="#000000" bordercolor="#000000" cellspacing="0" bordercolorlight="#000000" width="100%" border="0"> <tr> <td width="100%"> <table bgcolor="#FFFFFF" cellspacing="0" cellpadding="2" width="100%" border="0"> <tr> <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> </tr> <tr> <td valign="top"> </td><td valign="top"><span class="cstext"> <p>This seemingly simple example -- one router connecting two simple LANs -- must be fully understood if the student is to progress very far in their understanding of more complex and realistic networks. The router switches packets to the appropriate interface based on the destination IP address. It should also be noted that the router interfaces themselves must have addresses. A very appropriate kinesthetic activity is to have students play the roles of hosts and router and pass packets -- with Layer 2 and Layer 3 addresses -- back and forth across the network. The person portraying the router should remove the Layer 2 source address and replace it with its own.</p> <p>This TI relates to CCNA Certification Exam Objectives #7 and #36.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>Routers connect two or more networks. Each of which must have a unique network number in order for routing to be successful. The unique network number is incorporated into the IP address that is assigned to each device attached to that network.<img border="0" src="../../../../../CHAPID=knet-v214aCH47511/RLOID=knet-v214aRLO47824/RIOID=knet-v214aRIO123690/knet/v214adataimage1/1.gif" width="12" height="12"></p><p> <b>Example:</b> <br>A network has a unique network identifier, A. It has five devices attached to it. The IP addresses of the devices are A2, A3, A4, and A5. Since the interface where the router connects to a network is part of that network, the interface where the router connects to network A has an IP address of A1.<img border="0" src="../../../../../CHAPID=knet-v214aCH47511/RLOID=knet-v214aRLO47824/RIOID=knet-v214aRIO123690/knet/v214adataimage2/2.gif" width="12" height="12"></p><p> <b>Example:</b> <br>Another network, with a unique network identifier, B, has five devices attached to it. This network is also attached to the same router, but at a different interface. The IP addresses of the devices on this second network are B1, B2, B3, B4, and B5. The IP address of the router's second interface is B1.<img border="0" src="../../../../../CHAPID=knet-v214aCH47511/RLOID=knet-v214aRLO47824/RIOID=knet-v214aRIO123690/knet/v214adataimage3/3.gif" width="12" height="12"></p><p> <b>Example:</b> <br>You want to send data from one network to another. The source network is A, the destination network is B, and a router is connected to networks A, B, C, and D. When data (frames) coming from network A reaches the router, the router performs the following functions:</p><ol><li>It strips off the data link header, carried by the frame. (The data link header contains the MAC addresses of the source and destination.)</li><li>It examines the network layer address to determine the destination network.</li><li>It consults its routing tables to determine which of its interfaces it will use to send the data, in order for it to reach its destination network.</li></ol><p>In the example, the router determines that it should send the data from network A to network B, from its interface, with address B1. Before actually sending the data out interface B1, the router would encapsulate the data in the appropriate data link frame.</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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