IP assigned by ISP

["Palula Brasil" <palula () uol com br>]

Hi there!

I read on snort.conf that I can assign an IP address for my HOME_NET variable and if I desire, I can "use a global 
variable which will be always itialized to IP and netmask of the interface wich I run snort at".

That would be very nice for me because the IP address of my WAN NIC is assigned by my ISP.

The problem is that the syntax in snort.conf suggests: $(<interface_name>_ADDRESS) and at the bottom of that line it 
says: $eth0_ADDRESS

That looks pretty damn straight forward to me that I should put exactly: $eth0_ADDRESS at my configuration too. But 
when I run: /usr/sbin/snort -T, it fails ans gives this message:

ERROR: undefined variable name: (./snort.conf:45): eth0_ADDRESS
Fatal Error, Quitting..
And stops...

Can anybody hel me out?
Thanks! :)