Qtech QSW-3900-48-SFP-DC [148/245] Note the ip addresses offered by other dhcp servers if any are not used by the dhcp client and are still available to other clients

Qtech QSW-3900-48-SFP-DC [148/245] Note the ip addresses offered by other dhcp servers if any are not used by the dhcp client and are still available to other clients
QTECH Software Configuration Manual
11-147
· Manual assignment. The administrator statically binds IP addresses to few clients with special uses
(such as WWW server). Then the DHCP server assigns these fixed IP addresses to the clients.
· Automatic assignment. The DHCP server assigns IP addresses to DHCP clients. The IP addresses will
be occupied by the DHCP clients permanently.
· Dynamic assignment. The DHCP server assigns IP addresses to DHCP clients for predetermined
period of time. In this case, a DHCP client must apply for an IP address at the expiration of the period.
This policy applies to most clients.
11.2.9.2 Obtaining IP Addresses Dynamically
A DHCP client undergoes the following four phases to dynamically obtain an IP address from a DHCP server :
1) Discover : In this phase, the DHCP client tries to find a DHCP server by broadcasting a
DHCP-DISCOVER packet.
2) Offer : In this phase, the DHCP server offers an IP address. Each DHCP server that receives the
DHCP-DISCOVER packet chooses an unassigned IP address from the address pool based on the IP address
assignment policy and then sends a DHCP-OFFER packet (which carries the IP address and other configuration
information) to the DHCP client. The transmission mode depends on the flag field in the DHCP-DISCOVER packet.
For details, see section DHCP Packet Format.
3) Select : In this phase, the DHCP client selects an IP address. If more than one DHCP server sends
DHCP-OFFER packets to the DHCP client, the DHCP client only accepts the DHCP-OFFER packet that first arrives,
and then broadcasts a DHCP-REQUEST packet containing the assigned IP address carried in the DHCP-OFFER
packet.
4) Acknowledge : Upon receiving the DHCP-REQUEST packet, the DHCP server returns a DHCP-ACK
packet to the DHCP client to confirm the assignment of the IP address to the client, or returns a DHCP-NAK packet
to refuse the assignment of the IP address to the client. When the client receives the DHCP-ACK packet, it broadcasts
an ARP packet with the 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.
& Note : The IP addresses offered by other DHCP servers (if any) are not used by the
DHCP client and are still available to other clients.
11.2.9.3 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.
11.2.9.4 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) :

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