Visual Basic 1002005 [A Developers Notebook] [Electronic resources] نسخه متنی

6.2. Ping Another Computer

The
Internet is a dynamic network where computers appear and drop out of
sight without warning. One simple test an application can always
perform to check if a computer is reachable is to send a
ping message. Technically, a ping is the
equivalent of asking another computer, "Are you
there?" To get its answer, ping sends a special type
of message over a low-level Internet protocol called ICMP (Internet
Control Message Protocol).

Note: Need to find out if a computer is reachable over the
Internet? With the new Ping class, you can make this simple request
without a tangle of low-level socket code.

Sending a ping message using the classes found in the
System.Net namespaces is challenging and requires
dozens of low-level code statements that deal with raw sockets. In
.NET 2.0, there's a much simpler solution with the
new
Ping class in the
System.Net.NetworkInformation namespace.

6.2.1. How do I do that?

To ping a computer, you use the

Ping( ) method of the
My.Computer.Network object. This approach gives
you convenient access to the bare minimum ping functionality. The
Ping( ) method returns true or
False depending on whether it received a response
from the computer you're trying to contact.

Note: Windows includes a utility called ping.exe that you can use
to ping other computers at the command line.

Example 6-3 uses this method in order to contact the
web server at

Example 6-3. Pinging a remote computer

Module PingTest
Sub Main( )
Dim Success As Boolean
' Try to contact  (wait 1000 milliseconds at most,
' which is the default if you don't specify a timeout).
Success = My.Computer.Network.Ping(", 1000)
Console.WriteLine("Did the computer respond? " & Success)
End Sub
End Module

When you call Ping( ), you specify two parameters:
the URL or IP address for the computer you're trying
to reach (e.g., www.microsoft.com or
123.4.123.4) and, optionally, a maximum wait time in milliseconds.
Once this limit is reached, the request times out, and the
Ping( ) method returns False to
indicate the failure.

Warning: A ping message is a low-level test that doesn't
necessarily correspond to the availability of services on a
particular computer. For example, even if you can ping
, that doesn't
mean that its search engine web pages are available and working
properly. Similarly, web servers or firewalls often reject ping
messages to restrict the possibility of someone launching a
denial of service
attack by flooding the computer with millions of spurious requests.
For that reason, if you ping www.microsoft.com,
you won't receive a response, even though you can
still surf to their web site using that address.

6.2.2. What about...

...getting more information from the remote computer? The
My.Computer.Network object
doesn't return any additional information about the
results of the ping test. For example, you won't
find out how long it took to receive a response, which is a key
statistic used by some applications, such as peer-to-peer software,
to rank the connection speed of different computers.

To get more information, you need to head directly to the

Ping class in the
System.Net.NetworkInformation namespace. It
returns a PingResult object with
several pieces of information, including the time taken for a
response. The following code snippet puts this approach to the test.
It assumes that you've imported the
System.Net.NetworkInformation namespace:

Dim Pinger As New Ping
Dim Reply As PingReply = Pinger.Send(")
Console.WriteLine("Time (milliseconds): " & Reply.RoundTripTime)
Console.WriteLine("Exact status: " & Reply.Status.ToString( ))
Console.WriteLine("Adress contacted: " & Reply.Address.ToString( ))

Here's some sample output:

Time (milliseconds): 61
Exact status: Success
Adress contacted: 216.109.118.78

The Ping class also provides a
SendAsync( ) method you
can use to ping a computer without stalling your code (you can handle
the response in another thread when a callback fires), and other
overloaded versions of the Send( ) method that
allow you to set low-level options (like the number of hops the ping
message will travel before expiring).

6.2.3. Where can I learn more?

To use this added networking muscle, read up on the
Ping class in the MSDN Help.