54
•
Lease duration.
•
Gateway addresses.
•
Domain name suffix.
•
DNS server addresses.
•
WINS server addresses.
•
NetBIOS node type.
•
DHCP options.
Before assigning an IP address, the DHCP server performs IP address conflict detection to verify
that the IP address is not in use.
DHCP address pool
The DHCP server supports the following address assignment mechanisms:
•
Static address allocation
—Manually bind the MAC address or ID of a client to an IP address
in a DHCP address pool. When the client requests an IP address, the DHCP server assigns the
IP address in the static binding to the client.
•
Dynamic address allocation
—Specify IP address ranges in a DHCP address pool. Upon
receiving a DHCP request, the DHCP server dynamically selects an IP address from the
matching IP address range in the address pool.
You can specify the lease duration for IP addresses in the DHCP address pool.
The DHCP server observes the following principles to select an address pool for a client:
•
If there is an address pool where an IP address is statically bound to the MAC address or ID of
the client, the DHCP server selects this address pool and assigns the statically bound IP
address and other configuration parameters to the client.
•
If no static address pool is configured, the DHCP server selects an address pool depending on
the client location.
{
Client on the same subnet as the server
—The DHCP server compares the IP address of
the receiving interface with the subnets of all address pools. If a match is found, the server
selects the address pool with the longest-matching subnet.
{
Client on a different subnet than the server
—The DHCP server compares the IP
address in the
giaddr
field of the DHCP request with the subnets of all address pools. If a
match is found, the server selects the address pool with the longest-matching subnet.
IP address allocation sequence
The DHCP server selects an IP address for a client in the following sequence:
1.
IP address statically bound to the client's MAC address or ID.
2.
IP address that was ever assigned to the client.
3.
IP address designated by the Option 50 field in the DHCP-DISCOVER message sent by the
client. Option 50 is the Requested IP Address option. The client uses this option to specify the
wanted IP address in a DHCP-DISCOVER message. The content of Option 50 is user defined.
4.
First assignable IP address found in the way of selecting an address pool.
5.
IP address that was a conflict or passed its lease duration. If no IP address is assignable, the
server does not respond.
DHCP options
DHCP uses the options field to carry information for dynamic address allocation and provide
additional configuration information for clients.
You can customize options for the following purposes:
•
Add newly released DHCP options.
