HP VPN Firewall Appliances Network Management Configuration Guide

156
2. If the receiving interface has an extended address pool referenced, the DHCP server assigns an IP
address from this address pool. If no IP address is available in the address pool, the DHCP server
fails to assign an address to the client. For the configuration of such an address pool, see
"Configuring dynamic address allocation for an extended address p
ool."
3. Otherwise, the DHCP server selects the smallest common address pool that contains the IP address
of the receivin
g interface (if the client and the server reside on the same subnet), or the smallest
common address pool that contains the IP address specified in the giaddr field of the client's
request (if a DHCP relay agent is in-between). If no IP address is available in the address pool, the
DHCP server fails to assign an address to the client because it cannot assign an IP address from the
parent address pool to the client. For the configuration of such an address pool, see "Configuring
dynamic
address allocation."
For example
, two common address pools, 1.1.1.0/24 and 1.1.1.0/25, are configured on the DHCP server.
If the IP address of the interface receiving DHCP requests is 1.1.1.1/25, the DHCP server selects IP
addresses for clients from address pool 1.1.1.0/25. If no IP address is available in the address pool, the
DHCP server fails to assign addresses to clients. If the IP address of the interface receiving DHCP requests
is 1.1.1.130/25, the DHCP server selects IP addresses for clients from the 1.1.1.0/24 address pool.
NOTE:
To make sure IP address allocation functions correctly, keep the IP addresses for dynamic allocation within
the subnet where the interface of the DHCP server or DHCP relay agent resides.
IP address allocation sequence
A DHCP server assigns an IP address to 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 a DHCP-DISCOVER message.
Option 50 is the requested IP address field in DHCP-DISCOVER messages. It is padded by the
client to specify the IP address that the client wants to obtain. The contents to be padded depend
on the client.
4. First assignable IP address found in an extended or common 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 server configuration guidelines
DHCP networking involves two types:
• The DHCP server and clients are on the same subnet.
• The DHCP server and clients are not on the same subnet and communicate with each other through
a DHCP relay agent.
The DHCP server configuration for the two types is the same.