User's Manual

UTT Technologies Chapter 7 Advanced Setup
http://www.uttglobal.com Page 153
DHCP Client
DHCP Server
DHCPDISCOVER
DHCPOFFER
DHCPREQUEST
DHCPACK
Figure 7-21 Requesting for an IP Address from a DHCP Server
As shown in Figure 7-20, the process of a DHCP client requesting for an IP address from a
DHCP server falls into four basic phases:
z DHCP Discover: It is the phase that the DHCP client locates a DHCP server to ask
for an IP address.
The client broadcasts a DHCPDISCOVER message on its local
physical subnet. Only DHCP server(s) will respond it.
z DHCP Inform: It is the phase that one or more DHCP servers) offer an IP address to
the DHCP client. Once received the DHCPDISCOVER message, a DHCP server will
send a DHCPOFFER unicast message which includes configuration parameters
(such as an IP address, a domain name, a lease, and so on) to the DHCP client.
z DHCP Request: It is the phase that the DHCP client accepts the offer, chooses an IP
address and requests the address. The client may receive DHCPOFFER messages
from more than one DHCP server. Then the client chooses one from them, and
broadcasts a DHCPREQUEST message to formally request the offered IP address.
The DHCPREQUEST message also includes the server identifier option to indicate
which message it has selected, implicitly declining all other DHCPOFFER messages.
Once received the DHCPREQUEST message, those servers not selected will release the
IP addresses offered to the client.
If the configuration parameters sent to the client in the DHCPOFFER unicast
message by the DHCP Server are invalid (a misconfiguration error exists), the client
returns a DHCPDECLINE broadcast message to the DHCP Server to reject the
configuration assigned.
z DHCP Acknowledgement: It is the phase that the DHCP server officially assigns the
address to the client. Once received the DHCPREQUEST message, the selected
DHCP server will
respond with a DHCPACK unicast message containing the IP address
and other configuration parameters for the requesting client. Then the client will accept
and apply the IP address and other configuration parameters.
2. Renewing an IP Address
z An IP address dynamically allocated by a DHCP server for a client has a lease. The