H3C Technologies H3C S5560 Series Switches - Manual (Page 48)

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• If the IP address of the receiving interface is 1.1.1.130/25, the DHCP server selects IP addresses for
clients from the address pool 1.1.1.0/24.
It is a good practice to make sure the primary subnet can be matched so the DHCP server turns to the
secondary subnets only when the matching primary subnet has no assignable IP addresses. If only a
secondary subnet is matched, the DHCP server does not select any IP address from other secondary
subnets when the matching secondary subnet has no assignable addresses.
NOTE:
To make sure address allocation functions correctly, keep the IP addresses used for dynamic allocation in
the subnet where the interface of the DHCP server or DHCP relay agent resides if possible.
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 discussed in " DHCP 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.
NOTE:
If a client moves to another subnet, the DHCP server selects an IP address in the address pool matching the
new subnet instead of assigning the IP address that was once assigned to the client.
DHCP server configuration task list
Tasks at a glance
(Required.) Configuring an address pool on the DHCP server
(Required.) Enabling DHCP
(Required.) Enabling the DHCP server on an interface
(Optional.) Applying an address pool on an interface
(Optional.) Configuring IP address conflict detection
(Optional.) Enabling handling of Option 82
(Optional.) Configuring DHCP server compatibility
(Optional.) Setting the DSCP value for DHCP packets sent by the DHCP server