R3166-R3206-HP High-End Firewalls Network Management Configuration Guide-6PW101

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• 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 47 Dynamic IP address allocation process
As shown in Figure 47, 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.
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 assi
g
ned by the server
is in use by broadcastin
g
a
g
ratuitous 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.
IP address lease extension
The IP address dynamically allocated by a DHCP server to a client has a lease. When the lease elapses,
the IP address will be reclaimed by the DHCP server. If the client wants to use the IP address longer, it has
to extend the lease duration.
When the half lease duration elapses, the DHCP client sends to the DHCP server a DHCP-REQUEST
unicast message to extend the lease duration. Upon availability of the IP address, the DHCP server
returns a DHCP-ACK unicast confirming that the client’s lease duration has been extended, or a
DHCP-NAK unicast denying the request.
If the client receives no reply, it will broadcast another DHCP-REQUEST message for lease extension
when 7/8 lease duration elapses. The DHCP server will handle the request as mentioned.