The PIX Firewall does not allow you to assign a different IP address on an inside interface other than the range that is in use by the Dynamic Host Control Protocol (DHCP) server pool configured on the inside interface.
This is the error message observed:
Interface address is not on same subnet as DHCP pool
The address pool of a PIX Firewall DHCP server must be within the same subnet of the PIX interface that is enabled and you must specify the associated PIX interface with the if_name. In other words, the client must be physically connected to the subnet of a PIX interface.
Use the clear dhcpd command to clear the DHCP pool on the PIX.