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DHCP Configuration
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assigned IP address as the destination address to detect the assigned IP address, and uses
the IP address only if it does not receive any response within a specified period.
The IP addresses offered by other DHCP servers (if any) are not used by the DHCP client
and are still available to other clients.
(2) The next login for DHCP client
When DHCP client relogin, there are two types of situation:
The last-assigned IP address is not occupied. Broadcast a DHCP-REQUEST packet
containing the assigned IP address, the DHCP server returns a DHCP-ACK packet to the
DHCP client to confirm the assignment of the IP address to the client.
The last-assigned IP address is occupied. Broadcast a DHCP-REQUEST packet
containing the assigned IP address, the DHCP server returns a DHCP-NAK packet to refuse
the assignment of the IP address to the client. The client will re-send DHCP_Discover packet
to request a new IP address.
(3) Updating IP Address Lease
After a DHCP server dynamically assigns an IP address to a DHCP client, the IP address
keeps valid only within a specified lease time and will be reclaimed by the DHCP server when
the lease expires. If the DHCP client wants to use the IP address for a longer time, it must
update the IP lease.
By default, a DHCP client updates its IP address lease automatically by unicasting a
DHCP-REQUEST packet to the DHCP server when half of the lease time elapses. The DHCP
server responds with a DHCP-ACK packet to notify the DHCP client of a new IP lease if the
server can assign the same IP address to the client. Otherwise, the DHCP server responds
with a DHCP-NAK packet to notify the DHCP client that the IP address will be reclaimed when
the lease time expires.
If the DHCP client fails to update its IP address lease when half of the lease time elapses,
it will update its IP address lease by broadcasting a DHCP-REQUEST packet to the DHCP
server again when seven-eighths of the lease time elapses. The DHCP server performs the
same operations as those described in the previous section.
15.2.3 DHCP Packet Format
DHCP has eight types of packets. They have the same format, but the values of some
fields in the packets are different. The DHCP packet format is based on that of the BOOTP
packets. The following table describes the packet format (the number in the brackets indicates
the field length, in bytes):