Module Practice and Quiz

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DHCPv4 Concepts

The DHCPv4 server dynamically assigns, or leases, an IPv4 address to a client from a pool of addresses for a limited period of time chosen by the server, or until the client no longer needs the address. The DHCPv4 lease process begins with the client sending message requesting the services of a DHCP server. If there is a DHCPv4 server that receives the message it will respond with an IPv4 address and possible other network configuration information. The client must contact the DHCP server periodically to extend the lease. This lease mechanism ensures that clients that move or power off do not keep addresses that they no longer need. When the client boots (or otherwise wants to join a network), it begins a four-step process to obtain a lease: DHCPDISCOVER, then DHCPOFFER, then DHCPREQUEST, and finally DHCPACK. Prior to lease expiration, the client begins a two-step process to renew the lease with the DHCPv4 server: DHCPREQUEST then DHCPACK.

Configure a Cisco IOS DHCPv4 Server

A Cisco router running Cisco IOS software can be configured to act as a DHCPv4 server. Use the following steps to configure a Cisco IOS DHCPv4 server: exclude IPv4 addresses, define a DHCPv4 pool name, and configure the DHCPv4 pool. Verify your configuration using the show running-config | section dhcpshow ip dhcp binding, and show ip dhcp server statistics commands. The DHCPv4 service is enabled, by default. To disable the service, use the no service dhcp global configuration mode command. In a complex hierarchical network, enterprise servers are usually located centrally. These servers may provide DHCP, DNS, TFTP, and FTP services for the network. Network clients are not typically on the same subnet as those servers. In order to locate the servers and receive services, clients often use broadcast messages. A PC is attempting to acquire an IPv4 address from a DHCPv4 server using a broadcast message. If the router is not configured as a DHCPv4 server, it will not forward the broadcast. If the DHCPv4 server is located on a different network, the PC cannot receive an IP address using DHCP. The router must be configured to relay DHCPv4 messages to the DHCPv4 server. The network administrator releases all current IPv4 addressing information using the ipconfig /release command. Next, the network administrator attempts to renew the IPv4 addressing information with the ipconfig /renew command. A better solution is to configure R1 with the ip helper-address address interface configuration command. The network administrator can use the show ip interface command to verify the configuration. The PC is now able to acquire an IPv4 address from the DHCPv4 server as verified with the ipconfig /all command. By default, the ip helper-address command forwards the following eight UDP services:

  • Port 37: Time
  • Port 49: TACACS
  • Port 53: DNS
  • Port 67: DHCP/BOOTP server
  • Port 68: DHCP/BOOTP client
  • Port 69: TFTP
  • Port 137: NetBIOS name service
  • Port 138: NetBIOS datagram service

Configure a DHCPv4 Client

The Ethernet interface is used to connect to a cable or DSL modem. To configure an Ethernet interface as a DHCP client, use the ip address dhcp interface configuration mode command. Home routers are typically already set to receive IPv4 addressing information automatically from the ISP. The internet connection type is set to Automatic Configuration – DHCP. This selection is used when the router is connected to a DSL or cable modem and acts as a DHCPv4 client, requesting an IPv4 address from the ISP.


 

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