Static Mappings Inside DHCP Pools

While ISC dhcpd will allow a static mapping to be defined inside the DHCP range/pool, it can result in unexpected behavior.

ISC dhcpd only checks via ping to ensure that an IP is not actively in use when making assignments. Making a static mapping does not “reserve” that IP out of the pool. The static mapping in this case merely represents a preference for an IP and others are not prevented from taking the IP if it is not in use.

An example: If the DHCP pool is from to, and a static mapping is defined for If the PC that normally has is ever offline another device could be assigned When the other machine powers back up it will not be able to get because it is currently in use.

As such, it is best to only make assignments outside the range/pool, and the pfSense® webGUI enforces this practice.

If assignments absolutely must be made inside the pool, and the risks involved are worth taking and want to do so anyway, the input validation check may be removed from the PHP file that drives the DHCP editor page. The details of this unsupported change are left out as an exercise for the reader.