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Computers on our network not able to access internet suddenly

Posted on 2013-06-27
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Last Modified: 2013-07-03
For some inexplicable reason, many of the computers on our network stopped being able to access the internet. They did have connectivity to the internal network. We have two Win 2008 servers serving DHCP, One is serving 2-126, the other serving 127-254. We also have one of those servers also acting as the ADC. It seems that (with one exception) those people who have reserved IP's are able to connect to the internet just fine. However, many who have to lease a random IP are not. However, some can. We also have a guest Wi-Fi that should not be affected by this at all, because it is isolated from the internal network, and receiving DHCP directly from our Fortigate 80C firewall. However, at the same time as all of the problems with our network occured, the gues Wi-Fi lost access to the internet. At the moment, I am at a loss as to whether it is the DHCP servers, ADC server, or the Fortigate that is causing the problem. Or, some combination of the afore mentioned?
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Question by:jimstrickland
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by:ecarbone
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Here's are some things I would try:

1. Confirm that the SERVERS have static IP Addresses, and that they can access the Internet. Don't just see if IE's home page loads. Try browsing google.com and microsoft.com (or some other safe website (since you really shouldn't be browsing the web from your server anyway)).

2. Take note of the IP Address, Subnet Mask, Default Gateway and DNS settings on each server.

3. Look at the IP Configuration on one of the workstations that IS able to browse the Internet. (Again, test several sites to verify it is in fact able to browse the web).

4. Now look at the IP Configuration on one of the workstations that is NOT able to browse the web. From a command prompt, run (as administrator)  "ipconfig /all" and look for the IP Address of the active Ethernet adapter (or wireless adapter - however it is connecting to your internal network). If it has a valid IP Address (in the same subnet as your server and the workstation from step 3), try entering "ipconfig /releaseall" (or "ipconfig /release"), then enter "ipconfig /renew". When that finishes, take note of the new IP Address that is assigned. Again, check to see that it is in the same subnet as your server. If it is getting an auto assigned IP (beginning with 169.254...) then that means it is not getting an IP Address from any DHCP Server.

5. You mentioned both Servers are acting as DHCP Servers? Check to make sure the "DHCP Server" Service is running. (If both are off, any current leases will not notice so that may explain why some computers work and others dont)

6. If the DHCP Server Service IS running, check the settings and make sure it is assigning the correct IP information. (is it assigning the right gateway? is it assigning the right DNS?)

7. From the computer that cannot browse the internet ... from the command prompt, try pinging your server. Do you get a response? Ping your firewall. Do you get a response? Pint 127.0.0.1   Do you get a response?

8. Are you SURE the DHCP Server on your wireless router is not handing out IPs that overlap the IPs being handed out by your windows server?
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After several hours of banging our heads, we finally found the source of the problem. It turns out there was a "rogue" route in our Firewall routing table. We have no idea how the route got there, or even how long it has been there. We suspect it has been there for quite some time, causing minor issues here and there. Then, yesterday, our support was adjusting priorities in the routing table, trying to troubleshoot another issue, and it seemed to wake the sleeping monster in all of its furry. Once we zeroed in on the route, and deleted that record, the internet was accessible to all.
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by:jimstrickland
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I am choosing my own solution as the primary, because that is what fixed the problem. However, I am giving the points to the expert solution because he gave extremely good advice for someone initially facing internet issues on their network. I had already completed most of those steps before contacting Experts Exchange.
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