Comparing first 3 numbers of a IPv4 address?

Hello,

I'm trying to program a PHP-script, where users
can rate the "goodness" of the other players:

        create table pref_rep (
                id varchar(32) references pref_users(id) check (id <> author),
                author varchar(32) references pref_users(id),
                author_ip inet,
                good boolean,
                last_rated timestamp default current_timestamp
        );

To (try to) prevent tampering I'd like to delete
entries for the same id coming
from the same IP in the course of last hour:

        create or replace function pref_update_rep(_id varchar,
                _author varchar, _author_ip inet,
                _good boolean) returns void as $BODY$
                begin

                delete from pref_rep
                where id = _id and
                author_ip = _author_ip and
                age(to_timestamp(last_rated)) < interval '1 hour';

                update pref_rep set
                    author    = _author,
                    author_ip = _author_ip,
                    good      = _good,
                    last_rated = current_timestamp
                where id = _id and author = _author;

                if not found then
                        insert into pref_rep(id, author, author_ip, good)
                        values (_id, _author, _author_ip, _good);
                end if;
                end;
        $BODY$ language plpgsql;

I have 2 questions please:

1) if I'd like to compare just the first 3 numbers of
the IP address instead of the 4, how can I do it?
(yes, I know about the A,B,C type of IPv4 networks...)

2) Do I need to add an index to my table
or are id and author indexed already?

Thank you!
Alex