More than likely, your cable company won't let more than one MAC address through from your modem at a time.
As an aside...I've never heard of someone using USB and ethernet on their cable modem at the same time, and having it work. If I were you, I'd put the PC you currently have hooked up via USB onto the router with the others. Unplug everything from the cable modem, wait for an hour or so, and then plug the router back into it. That should be enough time for the MAC filtering your cable company is doing to reset.
thanks for ur advice,
but actually before provider did maintenance everything is OK, a computer using usb (straight to cable modem) and at the same time another 3 computer hooked to router (router hooked to cable modem), connection to internet is good.So i got problem after provider maitenance, a computer with usb is good but not with another 3 computer.
Now i tried what u told, all computer hooked to router but it didnt help. does it mean i got problem with router? and buy a new one ? how to make sure that router still run well
Power off the cable modem & router. Disconnect the USB cable from the cable modem and power on. Power on router and check the Internet connection on the 3 computers. Every cable mode I've ever seen will only provide 1 IP address unless you have static IPs.
There is one more option. Some ISP's don't like customers using routers and may even block mac address ranges designated to router manufacturers (it's rare, but there ar instances). If your router allows it, set it's MAC address to equal one of the PC NIC's plus 1. What I mean is if one PC has a mac of 0123.4567.89AB you set your routers public mac to 0123.4567.89AC. If they use mac address filtering that should solve your issue.
Cable modem hooks into the WAN port on your Router.
All 4 computers hook into the switch portion of your router (1-4/5).
Your cable modem will take 1-4 hours to pick up the new MAC address of your router.
Since you were using USB then swtiched to NIC in this instance, you might want to do this before you go to bed to give it time to kick in. Or you could call your cable company and tell them you switched from USB to a Network Card, at which time they'll manually reset their system to recognize your NIC over USB.