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+
+<head>
+<meta http-equiv="Content-Language" content="en-us">
+<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
+
+
+
+<title>Content</title>
+<base target="_self">
+</head>
+
+<body background="../../images/bg.gif" topmargin="0" leftmargin="0" marginheight="0" marginwidth="0" onLoad="window.focus();" link="#808080" vlink="#808080" alink="#808080">
+
+<table border="0" cellpadding="0" cellspacing="0" width="100%">
+ <tr>
+ <td bgcolor="#336666" width="18" valign="top">
+ <img border="0" src="../../images/content_lines.gif" width="16" height="25">
+ <img border="0" src="../../images/transdot.gif" width="2" height="1"></td>
+ <td bgcolor="#336666"><font face="Arial" size="2" color="#FFFFFF"><b>1.2</font></b></td>
+ <td bgcolor="#336666"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
+ <td bgcolor="#336666" width="100%"><b>
+ <font face="Arial" size="2" color="#FFFFFF">DHCP</font></b></td>
+ <td width="9" bgcolor="#336666"> </td>
+ </tr>
+ <tr>
+ <td bgcolor="#669999" height="25" width="18"> </td>
+ <td bgcolor="#669999" height="25"><b><font face="Arial" size="2" color="#FFFFFF">
+ 1.2.4</font></b></td>
+ <td bgcolor="#669999"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
+ <td bgcolor="#669999" height="25" width="100%"><b>
+ <font face="Arial" size="2" color="#FFFFFF">DHCP operation</font></b></td>
+ <td bgcolor="#669999" height="25" width="9"> </td>
+ </tr></table>
+
+<table border="0" cellpadding="0" cellspacing="0" width="95%" bordercolor="#111111">
+
+ <tr>
+ <td width="15"></td>
+ <td>
+ <p><font face="Arial" size="2">
+ The DHCP client configuration process uses the following steps: <img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12">
+ <img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"></p>
+ <ol>
+ <li>
+ A client must have DHCP configured when starting the network
+ membership process. The client sends a request to a server
+ requesting an IP configuration. Sometimes the client may suggest
+ the IP address it wants, such as when requesting an extension to a DHCP lease. The client locates a DHCP server by sending a broadcast
+ called a DHCPDISCOVER. </li>
+ <li>
+ When the server receives the broadcast, it determines whether it can
+ service the request from its own database. If it cannot, the server
+ may forward the request on to another DHCP server. If it can, the DHCP server offers the client IP
+ configuration information in the form of a unicast DHCPOFFER. The
+ DHCPOFFER is a proposed configuration that may include IP address,
+ DNS server address, and lease time.</li>
+ <li>
+ If the
+ client finds the offer agreeable, it will send another broadcast, a DHCPREQUEST, specifically requesting those particular IP parameters.
+ Why does the client broadcast the request instead of unicasting it
+ to the server? A broadcast is used because the first message, the DHCPDISCOVER, may have reached more than one DHCP server. If more
+ than one server makes an offer, the broadcasted DHCPREQUEST allows
+ the other servers to know which offer was accepted. The offer
+ accepted is usually the first offer received. </li>
+ <li>
+ The server that
+ receives the DHCPREQUEST makes the configuration official by sending
+ a unicast acknowledgment, the DHCPACK. It is possible, but highly
+ unlikely, that the server will not send the DHCPACK. This may happen
+ because the server may have leased that information to another
+ client in the interim. Receipt of the DHCPACK message enables the
+ client to begin using the assigned address immediately. </li>
+ <li>
+ If the client detects that the address is already in use on the
+ local segment it will send a DHCPDECLINE message and the process
+ starts again. If the client received a DHCPNACK from the server after
+ sending the DHCPREQUEST, then it will restart the process again.</li>
+ <li>
+ If the client no longer needs
+ the IP address, the client sends a DHCPRELEASE message to the
+ server.</font></li>
+ </ol>
+ <p><font face="Arial" size="2">
+ Depending on an organization's policies, it may be possible for an end
+ user or an administrator to statically assign a host an IP address
+ that belongs in the DHCP servers address pool. Just in case, the Cisco
+ IOS DHCP server always checks to make sure that an address is not in
+ use before the server offers it to a client. The server will issue an
+ ICMP echo request, or will ping, to a pool address before sending the
+ DHCPOFFER to a client. Although configurable, the default number of
+ pings used to check for a potential IP address conflict is two.</font></p>
+ <p>
+ <TABLE bgcolor="#B0AFAF" width="95%" border="0" cellspacing="0" cellpadding="0">
+ <TR>
+ <TD valign="top">
+ <TABLE bgcolor="#669999" width="100%" cellspacing="0" cellpadding="0" border="0">
+ <TR>
+ <TD width="5">
+ <img border="0" src="../../images/lab_toplft.gif" width="116" height="23"></TD>
+ <TD><IMG alt="" height="1" width="3" src="../../images/s.gif"></TD><TD align="right" valign="top">
+ <IMG alt="" src="../../images/corner_ur_7.gif" width="7" height="7"></TD>
+ </TR>
+ </TABLE>
+
+ <TABLE bgcolor="#B0AFAF" width="100%" cellspacing="0" cellpadding="0" border="0">
+ <TR>
+ <TD>
+ <TABLE width="100%" cellpadding="2" cellspacing="0" border="0" bordercolor="#111111">
+ <TR>
+ <TD bgcolor="#ffffff" width="15"> </TD>
+ <TD bgcolor="#ffffff"><font FACE="Arial" SIZE="2" COLOR="#000000">
+ <p><font color="#808080">
+ <img border="0" src="../../images/links_icon.gif" width="25" height="25">
+ <b>Web Links</b></font></p>
+ </font><font FACE="Arial" SIZE="2" COLOR="#808080">
+
+ <p>DHCP Operation<p>
+ <a target="_blank" href="http://www.cs.utexas.edu/users/ypraveen/surveys/DHCP/node2.html">
+ http://www.cs.utexas.edu/users/
+ ypraveen/surveys/ DHCP/ node2.html</a></font></p>
+
+ <p>
+ <IMG alt="" height="2" width="1" src="../../images/s.gif"></p>
+ </TD>
+ </TR>
+ </TABLE>
+ </TD>
+ </TR>
+ </TABLE>
+ </TD>
+ </TR>
+ </TABLE><p>
+
+ </td>
+ </tr>
+ </table>
+
+</body>
+
+</html>
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