Looks like I'm moving to "OS specified addresses" anyway, as I can't get my current impementation working properly. Using the implementation where I increment the local port for each bainaddress arg to the connectTCP call, I was receiving "connectbinderror" on the client whenever I closed and restarted a Server. Interstingly I would also get this error if I started the client only (Ie. no servers running which it could connect to). In my logs I would see the initial connection attempt, a failure due to "connection refused". After a backoff time a second connection attempt would be made where again it would fail but with a "connectbinderror" - "address already in use". My client factory is inheriting from the ReconnectingClientFactory. Using netstat on Windows I could see the client side socket listed as "LISTENING". Is this right for a client side socket when not actually connected? Anyway I guess that when the ReconnectingClientFactory attempted to perform a retry it would see the address as "in-use" and fail with a bind error similar to my original problem where I was trying to bind to an address which was already listening. When I let the OS specify the address I don't see this problem. Surely it is performing the same functions though...just with OS specified IP:Port combination? My app is working now with OS specified ports, though I would still like to understand where I was going wrong and why? Chris