31 Days Before Your CCNA Exam

Day 20 127■■■■■.au: Australia.co: Colombia.com: A business or industry.jp: Japan.org: A nonprofit organizationDHCPDHCP allows a host to obtain an IP address dynamically when it connects to the network. TheDHCP server is contacted by sending a request, and an IP address is requested. The DHCP serverchooses an address from a configured range of addresses called a pool and assigns it to the hostclient for a set period. Figure 20-4 graphically shows the process for how a DHCP server allocatesIP addressing information to a DHCP client.Figure 20-4Allocating IP Addressing Information Using DHCPDHCP ServerClientDHCPDISCOVER Broadcast1Unicast DHCPOFFER2 3DHCPREQUEST Broadcast“I have looked your offer over and I like it.”Unicast DHCPACK4“We are good to go! Here is your configuration.”When a DHCP-configured device boots up or connects to the network, the client broadcasts aDHCPDISCOVER packet to identify any available DHCP servers on the network. A DHCP serverreplies with a DHCPOFFER, which is a lease offer message with an assigned IP address, subnetmask, DNS server, and default gateway information, as well as the duration of the lease.The client can receive multiple DHCPOFFER packets if the local network has more than oneDHCP server. The client must choose between them and broadcast a DHCPREQUEST packet thatidentifies the explicit server and lease offer that it is accepting.Assuming that the IP address is still valid, the chosen server returns a DHCPACK (acknowledgment)message finalizing the lease. If the offer is no longer valid for some reason, the chosen serverresponds to the client with a DHCPNAK (negative acknowledgment) message. After it is leased,the client will renew prior to the lease expiration through another DHCPREQUEST. If the client ispowered down or taken off the network, the address is returned to the pool for reuse.