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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 first step in this process is identifying the IP address as class A (thus a default subnet mask of 255.0.0.0), class B (thus a default subnet mask of 255.255.0.0), or class C (thus a default subnet mask of 255.255.255.0). This establishes the "minimum" mask. The maximum mask must leave at least 2 bits for numbering hosts.</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>To create subnets, the routing portion of the address must be extended. The Internet knows a network as a whole, identified by the Class A, B, or C address, which defines 8, 16, or 24 routing bits (the network number). The subnet field will become additional routing bits, so that the routers within an organization can recognize different locations, or subnets, within the whole network.<img border="0" src="../../../../../CHAPID=knet-v214aCH47510/RLOID=knet-v214aRLO47818/RIOID=knet-v214aRIO123675/knet/v214adataimage1/1.gif" width="12" height="12"></p><ol><li> <b>Question</b>: In the address 131.108.0.0, which are the routing bits?<br><b>Answer</b>: 131.108 - That is the 16 bit Class B network number.</li><li> <b>Question</b>: What are the other two octets (16 bits) of the address 131.108.0.0 used for? <br><b>Answer</b>: As far as the Internet knows, that is just a 16 bit host field. This is because that is what a class B address is, a 16 bit network number and a 16 bit host number.</li><li> <b>Question</b>: What part of the address 131.108.0.0 is the subnet field? <br><b>Answer</b>: When you decide to create subnets, you must divide the original host field (16 bits in the case of Class B) into two parts - the subnet field and the host field. This is sometimes referred to as "borrowing" some of the original host bits to create the subnet field. The minimum number of bits that can be borrowed is 2, regardless of whether it is a Class A, B, or C network. Because at least 2 bits must remain for host numbers<sup>2</sup>, the maximum number of bits that can be borrowed varies by address class.</li></ol><table border="0" cellpadding="2" cellspacing="0"><tr><td align="center" colspan="1" rowspan="1" valign="top"><span class="smtext"><b>Address <br>Class</b></span></td><td align="center" colspan="1" rowspan="1" valign="top"><span class="smtext"><b>Size of Default<br>Host Field</b></span></td><td align="center" colspan="1" rowspan="1" valign="top"><span class="smtext"><b>Maximum Number <br>of Subnet Bits</b></span></td></tr><tr><td align="center" colspan="1" rowspan="1" valign="middle"><span class="smtext">A</span></td><td align="center" colspan="1" rowspan="1" valign="middle"><span class="smtext">24</span></td><td align="center" colspan="1" rowspan="1" valign="middle"><span class="smtext">22</span></td></tr><tr><td align="center" colspan="1" rowspan="1" valign="middle"><span class="smtext">B</span></td><td align="center" colspan="1" rowspan="1" valign="middle"><span class="smtext">16</span></td><td align="center" colspan="1" rowspan="1" valign="middle"><span class="smtext">14</span></td></tr><tr><td align="center" colspan="1" rowspan="1" valign="middle"><span class="smtext">C</span></td><td align="center" colspan="1" rowspan="1" valign="middle"><span class="smtext">8</span></td><td align="center" colspan="1" rowspan="1" valign="middle"><span class="smtext">6</span></td></tr></table><p>The subnet field always follows immediately after the network number. That is, the borrowed bits must be the first <i>n</i> bits of the default host field, where <i>n</i> is the desired size of the new subnet field.</p><p>The subnet mask is the tool used by the router to determine which bits are routing (network and subnet) bits and which bits are host bits.<img border="0" src="../../../../../CHAPID=knet-v214aCH47510/RLOID=knet-v214aRLO47818/RIOID=knet-v214aRIO123675/knet/v214adataimage2/2.gif" width="12" height="12"><img border="0" src="../../../../../CHAPID=knet-v214aCH47510/RLOID=knet-v214aRLO47818/RIOID=knet-v214aRIO123675/knet/v214adataimage3/3.gif" width="12" height="12"></p><p>Previous standards did not allow for the use of subnets obtained by borrowing 1 bit. With only 1 subnet bit, the subnet field can only have two values (that is, subnet 0 is part of the network address, and subnet 1 would be part of the network broadcast address). Although many devices can now support subnets obtained by borrowing 1 bit, it is still common practice to avoid doing this to insure compatibility with legacy devices. For our purposes here, you will always borrow at least 2 bits.</p><p>Similarly, a 1 bit host field would allow only for host 0, which is part of the network address, and host 1, which is part of the broadcast address, leaving 0 valid host addresses.</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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