H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 375)

35-2
When residing in a different subnet from the DHCP server, the DHCP client can get the IP address and
other configuration parameters from the server via a DHCP relay agent. For information about the
DHCP relay agent, refer to
Introduction to DHCP Relay Agent .
DHCP Address Allocation
Allocation Mechanisms
DHCP supports three mechanisms for IP address allocation.
z
Manual allocation: The network administrator assigns an IP address to a client like a WWW server,
and DHCP conveys the assigned address to the client.
z
Automatic allocation: DHCP assigns a permanent IP address to a client.
z
Dynamic allocation: DHCP assigns an IP address to a client for a limited period of time, which is
called a lease. Most clients obtain their addresses in this way.
Dynamic IP Address Allocation Process
Figure 35-2 Dynamic IP address allocation process
As shown in the figure above, a DHCP client obtains an IP address from a DHCP server via four steps:
1) The client broadcasts a DHCP-DISCOVER message to locate a DHCP server.
2) A DHCP server offers configuration parameters such as an IP address to the client in a
DHCP-OFFER message. The sending mode of the DHCP-OFFER is determined by the flag field in
the DHCP-DISCOVER message. Refer to
DHCP Message Format for related information.
3) If several DHCP servers send offers to the client, the client accepts the first received offer, and
broadcasts it in a DHCP-REQUEST message to formally request the IP address.