Index scan on CIDR field ?

I created index on cidr field on table with about 1 000 000 rows, made 
'vacuum analyze', but exlain told me that sequental scan is doneon query like that SELECT * FROM table WHERE
ipaddr='212.129.92.1'
 I'm using PostgreSQL 6.5.1. I need a index scan. Do I miss something ?
Margarit.