chances are you are trying to connect to a server that isnt linked/resolves incorrectly (some Random arent working). Check the website for the network you are trying to connect to for a list of linked servers.

be sure you arent blocking the server ports (6660-6669, 7000-7001, port 113 for identd) in router or firewall.

Use the search feature here, set it to all forums and expand the date range and you will see a lot of threads on "unable to connect"

ParaBrat @#mIRCAide DALnet