Dynamic Ip Address Allocation Process; Ip Address Lease Extension - HP 1910 User Manual

Dynamic IP address allocation process

Figure 279 Dynamic IP address allocation process
The client broadcasts a DHCP-DISCOVER message to locate a DHCP server.
1.
A DHCP server offers configuration parameters such as an IP address to the client in a
2.
DHCP-OFFER message. The sending mode of the DHCP-OFFER is determined by the flag field in
the DHCP-DISCOVER message. For more information about the DHCP message format, see
"DHCP message
If several DHCP servers send offers to the client, the client accepts the first received offer, and
3.
broadcasts it in a DHCP-REQUEST message to request the IP address formally.
All DHCP servers receive the DHCP-REQUEST message, but only the server from which the client
4.
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 unicast 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 in use. If the client receives no response within the specified time,
the client uses this IP address. Otherwise, the client sends a DHCP-DECLINE message to the server and
requests an IP address again.
IP addresses offered by other DHCP servers are still assignable to other clients.
•

IP address lease extension

The IP address dynamically allocated by a DHCP server to a client has a lease. When the lease expires,
the DHCP server will reclaim the IP address. To continue using the IP address, the client must extend the
lease duration.
After half the lease duration, the DHCP client sends a DHCP-REQUEST unicast to the DHCP server to
extend the lease. Depending on 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 after
7/8 lease duration elapses.
format."
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