H3C Technologies H3C SecPath F1000-E - Manual (Page 34)

2
DHCP Address Allocation
Allocation Mechanisms
DHCP supports three mechanisms for IP address allocation.
• 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.
• Automatic allocation: DHCP assigns a permanent IP address to a client.
• Dynamic allocation: DHCP assigns an IP address to a client for a limited period of time, which is
called a lease. Most DHCP clients obtain their addresses in this way.
Dynamic IP Address Allocation Process
Figure 2 Dynamic IP address allocation process
As shown in Figure 2 , 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. Upon receiving the message, a DHCP server offers configuration parameters including an IP
address to the client in a DHCP-OFFER message. The sending mode of the DHCP-OFFER message
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.
4. All DHCP servers receive the DHCP-REQUEST message, but only the server from which the client
accepts the offered IP address responds. The server returns a DHCP-ACK message to the client,
confirming that the IP address has been allocated to the client, or a DHCP-NAK message, denying
the IP address allocation.
NOTE:
•
After receiving the DHCP-ACK message, the client probes whether the IP address assigned by the server
is in use by broadcasting a gratuitous ARP packet. If the client receives no response within a specified
time, the client can use this IP address. Otherwise, the client sends a DHCP-DECLINE message to the
server and requests an IP address again.
• The IP addresses offered by other DHCP servers are still assignable to other clients.