H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 25)

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DHCP address allocation
Allocation mechanisms
DHCP supports the following mechanisms for IP address allocation.
• Static 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 7 Dynamic IP address allocation process
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. For related information, see “ 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 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 the client receives the DHCP-ACK message, it broadcasts a gratuitous ARP packet to verify whether
the IP address assigned by the server is already in use. If the client receives no response within the
specified time, the client uses the assigned IP address. Otherwise, the client sends a DHCP-DECLINE
message to the server to request an IP address again.
• IP addresses offered by other DHCP servers are still assignable to other clients.