planner not using index for like operator

For the query

Select col1 from table1

Where col1 like ‘172.%’

The table has 133 million unique ip addresses. Col1 is indexed.

The optimizer is using a sequential scan

This is the explain analyze output

"Seq Scan on table1 (cost=0.00..2529284.80 rows=1 width=15) (actual time=307591.339..565251.775 rows=524288 loops=1)"

"  Filter: ((col1)::text ~~ '172.%'::text)"

"Total runtime: 565501.873 ms"

The number of affected rows (500K) is a small fraction of the total row count.