In what way is it correct and in what way is it wrong?

Note that you are checking whether SYN packets reach the host and not ICMP packets.

the host is pingable by OS's "ping" command,but not pingable by this script.
but some hosts in other net is pingable by this script.

The ping command sends ICMP packets. Your code sends SYN packets and expects ACK replies.

I recommend you consult with your network administrator how to best check the connectivity between hosts on your network.

You can try to use Net::Ping::External, which actually invokes ping command.

Creation of ICMP-packets requires special privileges on some systems (e.g. suid on linux), so, it is a bit not-convenient to use ICMP-protocol from perl. ping command already has that privilege/capability, and it is more-or-less safe to use it from security point of view.

WBR, basiliscos.

Perhaps a firewall on the way to the host or the hosts own TCP stack configuration is interpreting your tests as a SYN-flood attack, consequently suppressing the ACK? What's the frequency of your tests?
I had no problems with Net::Ping in the past.

Update (in response to code block below): I second Corions advice given in Re^7: Net::ping is very unstable. (behold the use of [id://1122924] to point to that node ;-)

#!/usr/bin/perl -w
#use strict;

use warnings;

use IO::Handle;

use Socket;

use threads;

use IO::Socket::INET;

use Net::Ping;




open(pctxt,'./pc.txt')||die "error open $!";
open(pc_log, ">./pclog.txt")||die "error open $!";
my $stime = time;
while($read_data = <pctxt>)
{
    #chomp;
    $read_data=~s/\s//g;
    $read_data=~s/" "//g;
    thread($read_data);
}
my $etime = time;
my $diff = $etime-$stime;
print "$diff \n";
close(pc_log);
close(pctxt);

sub thread
{
    my $net = shift;
    my @ip = (1..255);
    my $port = 12345;
    my $thr0 = threads->new(\&scan, $net,@ip[0..15]);
    my $thr1 = threads->new(\&scan, $net,@ip[16..31]);
    my $thr2 = threads->new(\&scan, $net,@ip[32..47]);
    my $thr3 = threads->new(\&scan, $net,@ip[48..63]);
    my $thr4 = threads->new(\&scan, $net,@ip[64..79]);
    my $thr5 = threads->new(\&scan, $net,@ip[80..95]);
    my $thr6 = threads->new(\&scan, $net,@ip[96..111]);
    my $thr7 = threads->new(\&scan, $net,@ip[112..127]);
    my $thr8 = threads->new(\&scan, $net,@ip[128..143]);
    my $thr9 = threads->new(\&scan, $net,@ip[144..159]);
    my $thr10 = threads->new(\&scan, $net,@ip[160..175]);
    my $thr11 = threads->new(\&scan, $net,@ip[176..191]);
    my $thr12 = threads->new(\&scan, $net,@ip[192..207]);
    my $thr13 = threads->new(\&scan, $net,@ip[208..223]);
    my $thr14 = threads->new(\&scan, $net,@ip[224..239]);
    my $thr15 = threads->new(\&scan, $net,@ip[240..254]);
    $thr0->join();
    $thr1->join();
    $thr2->join();
    $thr3->join();
    $thr4->join();
    $thr5->join();
    $thr6->join();
    $thr7->join();
    $thr8->join();
    $thr9->join();
    $thr10->join();
    $thr11->join();
    $thr12->join();
    $thr13->join();
    $thr14->join();
    $thr15->join();
}


sub scan
{
    my ($net,@ip) = @_;
    foreach my $ip (@ip)
    {
      $host ="$net$ip";
      if (Is_Host_Alive($host))
      {
          my $socket = new IO::Socket::INET(
                PeerHost =>$host,
                PeerPort =>'12345',
                Proto    =>'tcp'
            );# or die "ERROR in Socket Creation : $!\n";
            if (($socket) && scan700($host))
            {
                print "$host not shutdown \n";
            }
        }
        #    $socket->close();
    }
}

sub scan700
{
    my ($ip) = @_;
  $host = "$ip";
  my $socket = new IO::Socket::INET(
            PeerHost =>$host,
            PeerPort =>'700',
            Proto    =>'tcp'
    );# or die "ERROR in Socket Creation : $!\n";
    if($socket)
        {
                print "$host is ATM \n";
                return 0;
        }
    #$socket->close();
    return 1;

}



sub Is_Host_Alive
{
    my ($host) = @_;
    my $tmp_icmp=Net::Ping->new('icmp');
    if(!$tmp_icmp->ping($host))
    {
           #print "$host ping unreachable \n";
        $tmp_icmp->close;
        return 0;
    }
    $tmp_icmp->close;
    return 1;
}
[download]

"pc.txt" file as following:
192.168.1.
192.168.2.
.....
192.168.150.
I want to check that whether so many net's hosts have been opened by port 12345 and not by port 700 or not.
Can you give me better advice for the result?

This post seems to be an exact duplicate of Re^6: Net::ping is very unstable (although I have to admit I haven't compared each and every character). If so, please don't duplicate posts in this way. It's possible and fairly convenient to embed a link to another node within any new node: please see What shortcuts can I use for linking to other information? and in particular How do I link to a node on this site by number?. If you post a node as a registered user and not anonymously, it's possible to edit your node at any time to add such a link (and much else as well!). If you wanted your current code to be accessible to Perlbotics via your reply rather than buried at the end of a long chain of other replies, it might have been better to have posted a reply that simply said "Please see Re^6: Net::ping is very unstable for my current complete code."

---

Give a man a fish:  <%-(-(-(-<