[boost] [ASIO] Host not found (authoritative)

Dear members, This is a very beginner's question on ASIO. I followed the tutorial example "daytime" in the ASIO document. Server one seems running well because nestat shows the port has been opened and listening. However, when I run the client one, the application prints out "Host not found (authoritative)" message only. I'm using port 21165. $ ./client localhost:21165 Is my command wrong? cheers, Alex Kim