Dynamic IP address assignment is used for assigning IP addresses to end-user devices, including workstations, Cisco IP phones, and mobile devices.
DHCP is used to provide dynamic IP address allocation to hosts. DHCP uses a client/server model; the DHCP server can be a Windows server, a UNIX-based server, or a Cisco IOS device. Cisco IOS devices can also be DHCP relay agents and DHCP clients. Figure 6-15 shows the steps that occur when a DHCP client requests an IP address from a DHCP server.
Step 1 The host sends a DHCPDISCOVER broadcast message to locate a DHCP server.
Step 2 A DHCP server offers configuration parameters such as an IP address, a MAC address, a domain name, a default gateway, and a lease for the IP address to the client in a DHCPOFFER unicast message.
Step 3 The client returns a formal request for the offered IP address to the DHCP server in a DHCPREQUEST broadcast message.
Step 4 The DHCP server confirms that the IP address has been allocated to the client by returning a DHCPACK unicast message to the client.
Figure 6-15 DHCP Operation
A DHCP relay agent is required to forward DCHP messages between clients and servers when they are on different broadcast domains (IP subnets). For example, the DHCP relay agent receives the DHCPDISCOVER message, which is sent as a broadcast, and forwards it to the DHCP server, on another subnet.
A DHCP client might receive offers from multiple DHCP servers and can accept any one of the offers; the client usually accepts the first offer it receives. An offer from the DHCP server is not a guarantee that the IP address will be allocated to the client; however, the server usually reserves the address until the client has had a chance to formally accept the address.
DHCP supports three possible address allocation mechanisms:
■ Manual: The network administrator assigns an IP address to a specific MAC address. DHCP is used to dispatch the assigned address to the host.
■ Automatic: DHCP permanently assigns the IP address to a host.
■ Dynamic: DHCP assigns the IP address to a host for a limited time (called a lease) or until the host explicitly releases the address. This mechanism supports automatic address reuse when the host to which the address has been assigned no longer needs the address.
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