1.2.5 Configuring DHCP Like NAT, a DHCP server requires that the administrator define a pool of addresses. The ip dhcp pool command defines which addresses will be assigned to hosts. The first command, ip dhcp pool , creates a pool with the specified name and puts the router in a specialized DHCP configuration mode. In this mode, use the network statement to define the range of addresses to be leased. If specific addresses on the network are to be excluded, return to global configuration mode. The ip dhcp excluded-address command configures the router to exclude an individual address or range of addresses when assigning addresses to clients. The ip dhcp excluded-address command may be used to reserve addresses that are statically assigned to key hosts, for instance, the interface address on the router. Typically, a DHCP server will be configured to assign much more than an IP address. Other IP configuration values such as the default gateway can be set from the DHCP config...
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1.2.4 DHCP operation The DHCP client configuration process uses the following steps: 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. 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. If the client finds the offer agreeable, it will send another broadcast, a DHCPREQUEST, spec...
1.2.2 BOOTP and DHCP differences The Internet community first developed the BOOTP protocol to enable configuration of diskless workstations. BOOTP was originally defined in RFC 951 in 1985. As the predecessor of DHCP, BOOTP shares some operational characteristics. Both protocols are client/server based and use UDP ports 67 and 68. Those ports are still known as BOOTP ports. The four basic IP parameters include: IP address Gateway address Subnet mask DNS server address BOOTP does not dynamically allocate IP addresses to a host. When a client requests an IP address, the BOOTP server searches a predefined table for an entry that matches the MAC address for the client. If an entry exists, then the corresponding IP address for that entry is returned to the client. This means that the binding between the MAC address and the IP address must have already been configured in the BOOTP server. There are two primary differences between DHCP and BOOTP: DHCP defines mechanism...
1.2.1 DHCP / Introducing DHCP Dynamic Host Configuration Protocol (DHCP) works in a client/server mode. DHCP enables DHCP clients on an IP network to obtain their configurations from a DHCP server. Less work is involved in managing an IP network when DHCP is used. The most significant configuration option the client receives from the server is its IP address. The DHCP protocol is described in RFC 2131. A DHCP client is included in most modern operating systems including the various Windows operating systems, Novell Netware, Sun Solaris, Linux, and MAC OS. The client requests addressing values from the network DHCP server . This server manages the allocation of the IP addresses and will answer configuration requests from clients. The DHCP server can answer requests for many subnets. DHCP is not intended for configuring routers, switches, and servers. These type of hosts need to have static IP addresses. DHCP works by providing a process for a server to allocate IP information to...
1.1.7 Issues with NAT NAT has several advantages, including: Conserves the legally registered addressing scheme by allowing the privatization of intranets. Increases the flexibility of connections to the public network. Multiple pools, backup pools, and load balancing pools can be implemented to assure reliable public network connections. Consistency of the internal network addressing scheme. On a network without private IP addresses and NAT, changing public IP addresses requires the renumbering of all hosts on the existing network. The costs of renumbering hosts can be significant. NAT allows the existing scheme to remain while supporting a new public addressing scheme. NAT is not without drawbacks. Enabling address translation will cause a loss of functionality, particularly with any protocol or application that involves sending IP address information inside the IP payload. This requires additional support by the NAT device. ...
1.1.5 Verifying PAT configuration Once NAT is configured, use the clear and show commands to verify that it is operating as expected. By default, dynamic address translations will time out from the NAT translation table after a period of non-use. When port translation is not configured, translation entries time out after 24 hours, unless the timers are reconfigured with the ip nat translation timeout timeout_ seconds command from global configuration mode. Clear the entries before the timeout by using one of the commands in Figure . Translation information may be displayed by performing one of the tasks in EXEC mode . Alternatively, use the show run command and look for NAT, access list, interface, or pool commands with the required values. 1.1.6 Troubleshooting NAT and PAT configuration When IP connectivity problems in a NAT environment exist, it is often difficult to determine the cause of the problem. Many times NAT is mistakenly blamed, when in reality ther...
1.1.4 Configuring NAT and PAT Static Translation To configure static inside source address translation, perform the tasks in Figures and . Figure shows the use of static NAT translation. The router will translate packets from host 10.1.1.2 to a source address of 192.168.1.2. Dynamic Translation To configure dynamic inside source address translation, perform the tasks in Figure . The access list must permit only those addresses that are to be translated. Remember that there is an implicit “deny all” at the end of each access list. An access list that is too permissive can lead to unpredictable results. Cisco advises against configuring access lists referenced by NAT commands with the permit any command. Using permit any can result in NAT consuming too many router resources, which can cause network problems. Figure translates all source addresses passing access list 1, which have source address from 10.1.0.0/24, to an address ...