Before installing and configuring my Active Directory, DNS and DHCP server,I manually assigned the NIC a static ip address (192.168.1.6). But after setting up the server, I can see the network icon on the system tray randomly flashing the blue light thing. But no network connection/traffic in and out of the server. Among the problems:
(1) I cannot ping, when I ping it says "Unknown Host"
(2) I cannot access the LAN
(3) I cannot even get to the internet - internet explorer kept saying "The Page cannot be displayed"

But befere these installation and setup (as stand alone server), my router assigned this machine an ip address at which time it accesses the internet with no single problem
The way out please??

This sounds like a gateway issue. I'd check the gateway setting.