F3726, F3211, F3174, R5135, R3816-HP Firewalls and UTM Devices Network Management Configuration Guide-6PW100

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482BDynamic IP address allocation process
Figure 81 Dynamic IP address allocation process
1. The client broadcasts a DHCP-DISCOVER message to locate a DHCP server.
2. Each 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 "
2339HDHCP 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 selected by the client
returns a DHCP-ACK message to confirm that the IP address has been allocated to the client, or a
DHCP-NAK message to deny the IP address allocation.
{ 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 can be assigned to other clients.
483BIP address lease extension
A dynamically assigned IP address has a lease. When the lease expires, the IP address is reclaimed by
the DHCP server. To continue using the IP address, the client must extend the lease duration.
When 1/2 lease duration elapses, the DHCP client unicasts a DHCP-REQUEST to the DHCP server to
extend the lease. Depending on the availability of the IP address, the DHCP server returns either 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 broadcasts another DHCP-REQUEST message for lease extension after
7/8 of the lease duration elapses. Again, depending on the availability of the IP address, the DHCP
server returns either a DHCP-ACK unicast confirming that the client's lease duration has been extended,
or a DHCP-NAK unicast denying the request.