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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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eval('top.frames.main_frame.location.href="'+ContentRef+'referencecontroller_crf.html"'); chapter = "knet-v214aCH47510"; ch_dash = chapter.indexOf('-'); ch_sub1 = chapter.substring(0,ch_dash); ch_sub2 = chapter.substring(ch_dash+1); params = "CHAPID=" + chapter + "/RLOID=" + "knet-v214aRLO47818" + "/RIOID=" + "knet-v214aRIO123678"; 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">10.7</td><td width="100%" class="rlohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="rlohdr">Creating a Subnet</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">10.7.4</td><td width="100%" class="riohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="riohdr">Computing hosts per subnetwork</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>There are several techniques for calculating the number of subnets when given the subnet mask and IP address. From the IP address, you can determine its class and hence the default subnet mask. Find how many bits beyond the default mask the actual subnet mask has been extended. This is the number of bits "borrowed" or stolen to create subnetworks.</p> <p>The formula 2<sup>m</sup>- 2, where m is the number of bits NOT stolen, gives the number of USABLE host numbers created. Another way to see this is to write out the powers of two, and find the exponent of two that matches the number of bits NOT stolen. Whatever that power of two equals (less the 2 reserved numbers) gives the number of hosts per subnetwork.</p> <p>This TI is related to CCNA Certification Exam Objectives #7, #29, #30, and #36.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>Each time 1 bit is borrowed from a host field, there is 1 less bit remaining in the field that can be used for host numbers. Specifically, each time another bit is borrowed from the host field, the number of host addresses that can be assigned decreases by a power of 2 (gets cut in half).</p><p>To help understand how this works, use a Class C network address as an example. If there is no subnet mask, all 8 bits in the last octet are used for the host field. Therefore, there are 256 (2<sup>8</sup>) possible addresses available to assign to hosts (254 usable addresses, after the 2 unusable addresses are subtracted). Now, imagine that this Class C network is divided into subnets. If 2 bits are borrowed from the default 8 bit host field, the host field decreases in size to 6 bits. If all of the possible combinations of 0s (zeros) and 1s that could occur in the remaining 6 bits were written out, the total number of possible hosts that could be assigned in each subnet would be reduced to 64 (2<sup>6</sup>). The number of usable host numbers would be reduced to 62.</p><p>In the same Class C network, if 3 bits were borrowed, the size of the host field decreases to 5 bits and the total number of hosts that could be assigned to each subnet would be reduced to 32 (2<sup>5</sup>). The number of usable host numbers would be reduced to 30.</p><p>The number of possible host addresses that can be assigned to a subnet is related to the number of subnets that have been created. In a Class C network, for example, if a subnet mask of 255.255.255.224 has been applied, then 3 bits (224 = 11100000) would have been borrowed from the host field. The useable subnets created are 6 (8 minus 2), each having 30 (32 minus 2) useable host addresses.</p><p> <b>Exercise:</b> <br>Divide the last octet into two parts, a subnet field and a host field. If there were 32 possible host addresses that can be assigned to each subnet, then their IP addresses would fall within the range of numbers. Remember the 2 unusable host addresses in each subnet.</p><p>In a Class C network 199.5.12.0 with subnet mask 255.255.255.224, to which subnet would host 199.5.12.97 belong? (hint: 97 = 01100001 binary)</p><ol type="A"><li>A. subnet 0?</li><li>B. subnet 1?</li><li>C. subnet 2?</li><li>D. subnet 3?</li><li>E. subnet 4?</li><li>F. none of the above?</li></ol><p> <b>Answer:</b> D. subnet 3.</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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