masking question

From: alexk (alexk_at_mailinator.com)
Date: 12/24/04 

Date: 24 Dec 2004 14:30:04 -0800

Hi,

I have the following table (it's from a book, not a real output):

Destination interface
0.1.0/18 eth0
0.2/12 eth1
etc

The (18) and (12) are masks, however, I don't understand to which part
of the address it applies. Does any one have a clue what the author
means ?
is 18 masked with the last 0, while 12 is maked with the 2 ?
Thank you great deal,
Alex