Ip Address Allocation Process; Ip Address Lease Extension - HP 6125XLG Layer 3-Ip Services Configuration Manual

IP address allocation process

Figure 11 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
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. (IP addresses
offered by other DHCP servers can be assigned to other clients.)
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 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 half of the 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 when
seven eighths 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.
"DHCP message
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format."