H3C Technologies H3C SecPath F1000-E - Manual (Page 229)

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 IP 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. 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. For details, refer to
DHCP Message Format .
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 unicast 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 clients.