Tuesday, 20 December 2011

Why the computer show "Limited Connectivity" ?

"Limited connectivity" happens when your computer can connect to the network ... but it can't.
I know, that wasn't very helpful. But it's actually accurate. Your computer was able to connect the network in one way, but was unable to complete the next step.

"Limited connectivity" happens when:
  • Your computer detects that a network is present and operating. That means that it detects that the network cable is plugged in, or that it was able to connect to a wireless access point.
  • Your computer's request for an IP address went unanswered.
It's that last one that needs a little explanation.
An IP address is how your computer is located on its network. The vast majority of machines are configured to connect to the network using DHCP to get what's called a "dynamic" IP address. That means that rather than permanently assigning a specific IP to that machine, the machine "asks" for an IP address when it connects to the network.
Dynamic IP addresses are simpler to configure, and allow for IP addresses to be re-used when the machines that they were temporarily assigned to are taken offline.
"If you check your IP on your machine and it starts with '169.' that's a made-up IP address."
A DHCP request for a dynamic IP address is very simple: your computer broadcasts to all the devices listening on its network "Would the DHCP server listening please assign me an IP address!". On each such network there should be one device that hears that request and responds with something like "Here ya go: you will be 192.168.1.4 - and by the way, when you want to talk to others on your network, here's some more information, and when you want to send something to the internet, send it here."
In other words your computer gets both its IP address as well as instructions on how to talk to the rest of the world.
At that point your computer is ready to communicate with the world.
So, what happens if no one responds?
Your computer will repeat that request ("Would the DHCP server listening please assign me an IP address!") a few times, and then it'll give up.
And declare that you have limited connectivity.
Connectivity is "limited" because your computer will likely make up an IP address. If you check your IP on your machine and it starts with "169." that's a made-up IP address. Only your computer knows about it, which means that no other computer knows how to contact yours. In addition, your computer doesn't know how to find or send packets to anyone else.
Limited connectivity is extremely limited. In practice, it means no connectivity.
There are scenarios where, if you know what you're doing and you're willing to jump through some hoops, you can actually force communication while you're in this state by doing things like manually configuring TCP/IP settings on your network card. But it's not meant to be a long term solution at all.
So, what do you do?
DHCP is typically handled by your router, or in some cases your broadband modem. If you have a single PC connected to the internet, it may also be handled by your ISP.
Believe it or not, 90% of the time what you did is exactly right: reboot your router. Sometimes routers just "get into a state" and need to be rebooted. It's silly, and in an ideal world shouldn't be needed, but it is what it is. I probably reboot my router once a month, on average.
Wireless access points, those which are not routers but simply provide the wireless connectivity to a wired network, can sometimes also hiccup and fail in such a way as to cause this problem. I believe it happens when the wireless side of the device keeps working while the wired connection is, for some reason, inoperative. Again, a reboot of the device typically clears this up.
In either case, it's easiest to then reboot your computer(s) to force them to request new IP address assignments from the now functioning router.
If the reboot resolves the problem, but then it reoccurs quickly, I would check with the manufacturer of that device to see if there is new firmware available for it. Even though we think of routers and access points as hardware, they are in fact small computers running programs, and the programs sometimes have bugs. Sometimes those bugs don't manifest until something outside of the device changes. (I've heard anecdotal evidence that the new TCP/IP code in Windows Vista has exposed issues with some routers, for example.)
If your firmware is up to date, and the problem repeats frequently, I would investigate trying a new router or access point to see if that clears things up.

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