How do I prevent Socket/Port Exhaustion?

I am attempting to performance test a website by hitting it with requests across multiple threads. Each thread executes n times. (in a for loop)

However, I am running into problems. Specifically the WebException ("Unable to connect to remote server") with the inner exception:

An operation on a socket could not be performed because the system lacked sufficient buffer space or because a queue was full 127.0.0.1:52395

I am attempting to run 100 threads at 500 iterations per thread.

Initially I was using HttpWebRequest in System.Net to make the GET request to the server. Currently I am using WebClient as I assumed that each iteration was using a new socket (so 100 * 500 sockets in a short period of time). I assumed WebClient (which is instantiated once per thread) would only use one socket.

I don't need 50 000 sockets open at once, as I would like to send the GET request, receive the response, and close the socket, freeing it for use in the next loop iteration. I understand that it would be a problem to

However, even with WebClient, a bunch of sockets are being requested resulting in a bunch of sockets in TIME_WAIT mode (checked using netstat). This causes other applications (like internet browsers) to hang and stop functioning.

I can operate my test with less iterations and/or less threads, as it appears the sockets do eventually exit this TIME_WAIT state. However, this is not a solution as it doesn't adequately test the abilities of the web server.

Question:

How do I explicitly close a socket (from the client side) after each thread iteration in order to prevent TIME_WAIT states and socket exhaustion?

Code:

Class that wraps the HttpRequest

Edit: Wrapped WebClient in a using, so a new one is instantiated,used and disposed for every iteration. The problem still persists.

  public sealed class HttpGetTest : ITest {
    private readonly string m_url;

    public HttpGetTest( string url ) {          
        m_url = url;
    }

    void ITest.Execute() {
        using (WebClient webClient = new WebClient()){
            using( Stream stream = webClient.OpenRead( m_url ) ) {          
            }
        }
    }
}

The part of my ThreadWrapperClass that creates a new thread:

public void Execute() {
    Action Hammer = () => {
        for( int i = 1; i <= m_iterations; i++ ) {
            //Where m_test is an ITest injected through constructor
            m_test.Execute();
        }       
    };
    ThreadStart work = delegate {
        Hammer();
    };
    Thread thread = new Thread( work );
    thread.Start();
}