select like and indexes

Hi,

I'm quite new to postgresql. I need some help to understand some of
'how it works'.

I have a table with aprox. 1 million tuples (row == tuple ??) and
I've created an index on a column 'ip' in my table.

Well the question is: when I do,

explain analyze select * from tbl_xxxx where ip like '200%';

why PG doesn't use Index Scan method? why does it uses sequential
scan in place?

Thank you,

william