"Failed to connect to server" message

[Ian Thorpe]

I have a PC behind a firewall router then I'm trying (unsuccessfully!) to
connect to.  I have set up the port forwarding and both
http://www.realvnc.com/cgi-bin/nettest.cgi and http://www.gotomyvnc.com/ say
that the (external) ip and port 5900 are accepting connections and get a reply
from the VNC server.  However telnet can't connect on that port and the VNC
viewer comes straight back with "Failed to connect to server".   What am I
doing wrong??

Ian