$addr = "select * from address_tbl where Subnet=?";
$subs = "select * from subnet_tbl where VLAN=?";
$addr_str = $dbh->prepare($addr); # prepare address select
$subs_str = $dbh->prepare($subs); # prepare subnet select

#sth is the statement handle from your original query
while(@row = $sth->fetchrow_array()) {
   $addr_str->execute($row[0]);
}