Given a range of IP addresses entered by a user (through various means), I want to identify which of these machines have software running that I can talk to.

Here's the basic process:

1. Ping these addresses to find available machines

2. Connect to a known socket on the available machines

3. Send a message to the successfully established sockets

4. Compare the response to the expected response

Steps 2-4 are straight forward for me. What is the best way to implement the first step in .NET?

I'm looking at the System.Net.NetworkInformation.Ping class. Should I ping multiple addresses simultaneously to speed up the process? If I ping one address at a time with a long timeout it could take forever. But with a small timeout, I may miss some machines that are available.

Sometimes pings appear to be failing even when I know that the address points to an active machine. Do I need to ping twice in the event of the request getting discarded?

To top it all off, when I scan large collections of addresses with the network cable unplugged, Ping throws a NullReferenceException in FreeUnmanagedResources(). !?

Any pointers on the best approach to scanning a range of IPs like this?