Whenever a DHCP client attempts to lease an IP address and
the DHCP server cannot provide an address, this variable is
set to the gateway address of the DHCP request. The gateway
address is the IP address of the gateway acting as a BOOTP
relay agent for the request. If there is no gateway, the
address will be 0.0.0.0. If the gateway address is non-zero,
the DHCP range from which the allocation should have been
made will able to be identified by lookup in the DHCP range
table.