DHCP Configuration
addresses will be occupied by the DHCP clients permanently.
Dynamic assignment. The DHCP server assigns IP addresses to DHCP clients for
predetermined period of time. In this case, a DHCP client must apply for an IP address at the
expiration of the period. This policy applies to most clients.
15.2.2 Obtaining IP Addresses Dynamically
Interaction between a DHCP client and a DHCP server is as shown in Picture 1-2.
Figure 15-2 Interaction between a DHCP client and a DHCP server
There are three different modes for DHCP client to obtain IP address in different stage:
(1) Initial login for DHCP client
There are four stages for the initial login of DHCP client:
Discover. In this phase, the DHCP client tries to find a DHCP server by broadcasting a
DHCP-DISCOVER packet.
Offer. In this phase, the DHCP server offers an IP address. Each DHCP server that
receives the DHCP-DISCOVER packet chooses an unassigned IP address from the address
pool based on the IP address assignment policy and then broadcasts a DHCP-OFFER packet
to the DHCP client.
Select: In this phase, the DHCP client selects an IP address. If more than one DHCP
server sends DHCP-OFFER packets to the DHCP client, the DHCP client only accepts the
DHCP-OFFER packet that first arrives, and then broadcasts a DHCP-REQUEST packet
containing the assigned IP address carried in the DHCP-OFFER packet.
Acknowledge: Upon receiving the DHCP-REQUEST packet, the DHCP server returns a
DHCP-ACK packet to the DHCP client to confirm the assignment of the IP address to the
client. When the client receives the DHCP-ACK packet, it broadcasts an ARP packet with the
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