It is hard to manage a large network. For example, in a network in which IP addresses are manually allocated, IP address conflicts are common. The only way of solving the problem is to dynamically allocate IP addresses to the hosts. The Dynamic Host Configuration Protocol (DHCP) allocates IP address to requesting hosts from an IP address pool. DHCP also provides other information, such as gateway IP and DNS server address. DHCP reduces the workload of the administrator in recording and tracking manually allocated IP addresses.
DHCP is a protocol that is based on UDP broadcast. The process for a DHCP client to obtain an IP address and other configuration information contains four phases.
DISCOVER phase. When the DHCP client accesses the network for the first time, it sends a DHCP DISCOVER packet with the source address 0.0.0.0 and destination address 255.255.255.255 to the network.
OFFER phase. After the DHCP server receives the DHCP DISCOVER broadcast packet sent by the client, it selects an IP address from the corresponding IP address pool according to the policy, and sends the IP address and other parameters to the client in a DHCP OFFER packet.
REQUEST phase. If the DHCP client receives response messages from multiple DHCP servers on the network, it selects one DHCP OFFER (usually the one that arrives first). Then it sends a DHCP REQUEST packet to the network, telling all DHCP servers the IP address of which server it will accept.
ACK phase. After the DHCP server receives the DHCP REQUEST packet from the DHCP client, it sends a DHCP ACK message containing the provided IP address and other configuration to the DHCP client, telling the DHCP client that the DHCP client can use the provided IP address.
The IP address that the DHCP server allocates to the DHCP client has a lease. After the lease expires, the server will take back the allocated IP address. When the lease term of the IP address of the DHCP client has passed half time, the DHCP client sends a DHCP REQUEST packet to the DHCP server requesting to update its IP address lease. If the DHCP server allows the DHCP client to use its IP address, the DHCP server responds with a DHCP ACK packet, requesting the DHCP client to update the lease. If the DHCP server does not allow the DHCP client to continue to use the IP address, the DHCP server responds with a DHCP NAK packet.
During dynamic IP address acquisition, request packets are sent in broadcast mode; therefore, DHCP is applied only when the DHCP client and server are in the same subnet. If multiple subnets exist in a network and the hosts of the subnets need to obtain configuration information such as IP address through the DHCP server, the hosts of the subnets communicate with the DHCP server through a DHCP relay to obtain IP addresses and other configuration information.
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