seq scan on indexed column

Hi,
I always have questions on sql tunning, here is the one:

gtld_analysis=# \d gtld_owner

Table "gtld_owner"
Attribute | Type | Modifier
----------------+------------------------+------------
owner_name | character varying(100) |
netblock_start | integer | not null /* primary key */
netblock_end | integer | not null
country | character varying(100) |
country_c | smallint | default -1
region | character varying(100) |
region_c | smallint | default -1
city | character varying(100) |
city_c | smallint | default -1
lat | numeric(9,4) |
long | numeric(9,4) |
Indices: gtld_owner_pkey,
owner_nb_end

gtld_analysis=# select count(*) from gtld_owner;
count
---------
2174335
(1 row)

gtld_analysis=# explain SELECT NETBLOCK_START
gtld_analysis-# FROM GTLD_OWNER
gtld_analysis-# WHERE NETBLOCK_START = -2147483648;
NOTICE: QUERY PLAN:

Seq Scan on gtld_owner (cost=0.00..80021.37 rows=23 width=4)

EXPLAIN

Why it didn't use index scan? what rows=23 means? We have an application
which loops each row and select netblock_start, and it is slow overall.

Anyone can give me a clue? Thanks!

Anna Zhang