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The most Clients obtain the IP addresses dynamically, which is illustrated in the following
figure.
Figure 12-6 Interaction between a DHCP client and a DHCP server
(1) DHCP-DISCOVER Stage: The Client broadcasts the DHCP-DISCOVER packet to find
the DHCP Server.
(2) DHCP-OFFER Stage: Upon receiving the DHCP-DISCOVER packet, the DHCP Server
selects an IP address from the IP pool according to the assigning priority of the IP
addresses and replies to the Client with DHCP-OFFER packet carrying the IP address
and other information.
(3) DHCP-REQUEST Stage: In the situation that there are several DHCP Servers sending
the DHCP-OFFER packets, the Client will only respond to the first received
DHCP-OFFER packet and broadcast the DHCP-REQUEST packet which includes the
assigned IP address of the DHCP-OFFER packet.
(4) DHCP-ACK Stage: Since the DHCP-REQUEST packet is broadcasted, all DHCP Servers
on the network segment can receive it. However, only the requested Server processes
the request. If the DHCP Server acknowledges assigning this IP address to the Client, it
will send the DHCP-ACK packet back to the Client. Otherwise, the Server will send the
DHCP-NAK packet to refuse assigning this IP address to the Client.
Option 82
The DHCP packets are classified into 8 types with the same format basing on the format of
BOOTP packet. The difference between DHCP packet and BOOTP packet is the Option field.
The Option field of the DHCP packet is used to expand the function, for example, the DHCP can
transmit the control information and network parameters via the Option field, so as to assign
the IP address to the Client dynamically. For the details of the DHCP Option, please refer to RFC
2132.
Option 82 records the location of the DHCP Client. Upon receiving the DHCP-REQUEST packet,
the switch adds the Option 82 to the packet and then transmits the packet to DHCP Server.
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