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<html> <head> <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"> var pageLoaded = false; var current = ''; var currentChapter = 'knet-v214aCH47510'; 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_pf.html"'); } function PrevPage() { top.currentSupport--; if (top.currentSupport <= 0) { returnContent(); } else { if (top.firstSupportPage == top.contentResourceID) { returnContent(); } else if (top.prevPage == quizID) { launchQuiz(); } else { launchReview(); } } } function launchReview(reviewID) { top.currentSupport = 0; chapterID = "knet-v214aCH47510"; nbparams = "CHAPID="+chapterID+"/RLOID="+"knet-v214aRLO47818"+"/RIOID="+reviewID; dash = chapterID.indexOf('-'); sub1 = chapterID.substring(0,dash); sub2 = chapterID.substring(dash+1); ContentRef = "../../../../../"+ nbparams +"/"+ sub1 +"/"+ sub2 +"/"; //alert(ContentRef); eval('top.frames.main_frame.location.href="'+ContentRef+'pageframesetrio.html"'); //added for navigating between Quiz and Review and back to regular Chapter pages top.currentSupport++; top.firstSupportPage = reviewID; top.prevPage = reviewID; top.contentResourceID = ''; } function launchQuiz(quizID) { top.currentSupport = 0; chapterID = "knet-v214aCH47510"; nbparams = "CHAPID="+chapterID+"/RLOID="+"knet-v214aRLO47821"+"/RIOID="+"knet-v214aRIO123683"; dash = chapterID.indexOf('-'); sub1 = chapterID.substring(0,dash); sub2 = chapterID.substring(dash+1); ContentRef = "../../../../../"+ nbparams +"/"+ sub1 +"/"+ sub2 +"/"; //alert(ContentRef); eval('top.frames.main_frame.location.href="'+ContentRef+'pageframesetrio.html"'); //added for navigating between Quiz and Review and back to regular Chapter pages top.currentSupport++; top.firstSupportPage = quizID; top.prevPage = quizID; top.contentResourceID = ''; } function returnContent() { rioID = top.currentPage; rloID = top.currentRLO; ContentID = top.currentChapter; top.firstSupportPage = ''; top.prevPage = ''; top.contentResourceID = ''; top.currentSupport = 0; nbparams = "CHAPID="+ContentID+"/RLOID="+rloID+"/RIOID="+rioID; dash = ContentID.indexOf('-'); sub1 = ContentID.substring(0,dash); sub2 = ContentID.substring(dash+1); ContentRef = "../../../../../"+ nbparams +"/"+ sub1 +"/"+ sub2 +"/" eval('top.frames.main_frame.location.href="'+ContentRef+'pageframesetrio.html"'); } </script> </head> <body leftmargin="20" topmargin="10" marginwidth="30" marginheight="10" bgcolor="#FFFFFF" onLoad="window.focus();" background="../../../../../images/ccna/common/lab/bglab.gif"> <table bgcolor="#000000" cellspacing="0" width="100%" border="0"> <tr> <td width="100%"> <table cellpadding="4" cellspacing="1" width="100%" border="0"> <tr> <td width="10%" bgcolor="#FFFFFF" valign="top"> <table cellpadding="0" cellspacing="0" width="100%" border="0"> <tr> <td width="33%"><script language="JavaScript"> //if (parent.parent.getTopName() == "chapterframeset") //{ var imgSource = '../../../../../images/ccna/common/lab/labback.gif'; document.write('<a href="javascript:PrevPage();">'); document.write('<img src="' + imgSource + '" border="0">'); document.write('</a>'); //} </script></td><td width="33%"></td> </tr> </table> <p> </p> </td><td bgcolor="#FFFFFF" valign="top"> <p align="left"> <span class="resourceTitle">10.7.5 Boolean AND operation</span> </p><object height="100%" width="100%" codebase="http://active.macromedia.com/flash4/cabs/swflash.cab#version=4,0,0,0" classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000"><param name="movie" value="../../../../../CHAPID=knet-v214aCH47510/RLOID=knet-v214aRLO47818/RIOID=knet-v214aRIO123679/knet/v214aVID4936/10_7_5.swf"><param value="high" name="quality"> <embed height="100%" width="100%" pluginspage="http://www.macromedia.com/shockwave/download/index.cgi?P1_Prod_Version=ShockwaveFlash" type="application/x-shockwave-flash" quality="high" src="../../../../../CHAPID=knet-v214aCH47510/RLOID=knet-v214aRLO47818/RIOID=knet-v214aRIO123679/knet/v214aVID4936/10_7_5.swf"> </embed> </object>Instructor Note<p class="smtext">The key to understanding the result of the Boolean ANDing of an IP address and a subnet mask is to realize that once created, subnetworks are valid network numbers as far as the "outside" world is concerned. So as with the earlier calculation, the bit-by-bit ANDing of the IP address and the subnet mask gives the subnetwork number.</p><p class="smtext">The "lab", a paper-based activity, requires approximately 45 minutes. This TI is related to CCNA Certification Exam Objectives #7, #29, #30, and #36.</p><p class="smtext">The lowest numbered address in an IP network is the network address (the network number plus 0 in the entire host field). This also applies to a subnet. The lowest numbered address is the address of the subnet.</p><a href="#" onClick="javascript:launchReference('knet-v214aRIO125375', 'null', 'null');void(null);">Lab Activity</a><p class="smtext">This lab focuses on a Class B network with three subnets. It will help you develop a better understanding of IP subnet masks.</p><p class="smtext">In order to route a data packet, the router must first determine the destination network or subnet address. To accomplish this the router performs a logical AND using the destination host's IP address and the subnet mask for that network.</p><p class="smtext">Imagine a Class B network with the network number 172.16.0.0. After assessing the needs of the network, a decision is made to borrow 8 bits in order to create subnets. When 8 bits are borrowed with a Class B network, the subnet mask is 255.255.255.0.<img border="0" src="../../../../../CHAPID=knet-v214aCH47510/RLOID=knet-v214aRLO47818/RIOID=knet-v214aRIO123679/knet/v214adataimage1/1.gif" width="12" height="12"></p><p class="smtext">Someone outside the network sends data to the IP address 172.16.2.120. In order to determine where to deliver the data, the router ANDs this address with the subnet mask. When the two numbers are ANDed, the host portion of the result will always be 0. The network number is left, including the subnet. Thus, the data is sent to subnet 172.16.2.0, and only the final router notices that the packet should be delivered to host 120 in that subnet.</p><p class="smtext">Now, imagine the same network, 172.16.0.0. This time, however, only 7 bits were borrowed for the subnet field. The binary subnet mask for this would be 11111111.11111111.11111110.00000000. What would this be in dotted decimal notation?</p><p class="smtext">Again, someone outside the network sends data to host 172.16.2.120. In order to determine where to send the data, the router again ANDs this address with the subnet mask. As before, when the two numbers are ANDed, the host portion of the result is 0. So what is different in this second example? Everything looks the same, at least in decimal. The difference is in the number of subnets available, and the number of hosts that can be in each subnet. This can only be seen by comparing the two different subnet masks.<img border="0" src="../../../../../CHAPID=knet-v214aCH47510/RLOID=knet-v214aRLO47818/RIOID=knet-v214aRIO123679/knet/v214adataimage2/2.gif" width="12" height="12"></p><p class="smtext">With 7 bits in the subnet field, there can be only 126 subnets. How many hosts can there be in each subnet? How long is the host field? With 9 bits for host numbers, there can be 510 hosts in each of those 126 subnets.</p><a href="#" onClick="window.open('../../../../../CHAPID=null/RLOID=null/RIOID=knet-v214aRIO123679/knet/v214aRIOv2141005/iaactivity.html','LabActivity','width=742,height=390,toolbar=0,scrollbars=0,left=26,top=38');void(null);">Interactive Lab Activity</a><p class="smtext">Subnetting Class B IP Addresses</p>Web Links<a target="_blank" href="http://www.howstuffworks.com/boolean.htm"><span class="smtext">How Boolean Logic Works</span></a><p class="smtext">The two graphics on this page include an alternate way to express the subnet mask. The 1s of the mask represent the routing bits, the network plus the subnet. 255.255.255.0 indicates there are 24 total routing bits. This is sometimes indicated by following an IP address with "/24", as in 131.108.3.1 /24. This says the same thing as the longer subnet mask.</p></td> </tr> </table> </td> </tr> </table> </body></html>⌨️ 快捷键说明
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