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I can't access some websites from a subset of computers on our work network - and it's really random. Examples of sites I can't access:

Our provider (Verizon) swears that everything is good on their end. We've tried rebooting the router - to no avail. Out of 10 PCs in the office, 4 of them can't access the sites above.

I've tried comparing tracert between a site that I can and cannot access. The only meaningful difference I see is that after leaving the Verizon network, the non-accessible sites go through, while accessible ones do not.

Here is the tracert from hitting (accessible) [] [] []
0.ae3.XL3.LAX15.ALTER.NET []
TenGigE0-4-2-0.GW4.LAX15.ALTER.NET [] [] []

Here is the tracert from hitting (non-accessible) [] []
 0.xe-4-1-0.BR1.LAX15.ALTER.NET [] [] []
Request timed out.

The problem started occurring sometimes last week. How can I troubleshoot it?
P.S. The problematic machines are a mix of Win7 and XP (as are the non-problematic ones)

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I don't know what's causing it however I can tell you that as long as 6 of the PCs can access the website, it's an internal error of the computers, not Verizon. – Frank Jun 22 '13 at 3:51
can you show us ROUTE PRINT output from problematic ones? – Dusan Bajic Jun 22 '13 at 7:12

This problem does not lie with your ISP. Much to user Chipperyman's point, If any of the machines can access the aforementioned sites, then the issue lies inside of the local network (work) or the individual client machines.

Unfortunately, without more visibility into the client machines or the network they lie on, not much can be provided in the way of true solutions.

A recommendation of troubleshooting steps would be:

  1. Verify the DNS and Default Gateway settings of machines that can hit the sites versus those than cannot.
  2. Verify that the "hosts" file of each client machine does not contain strange entries.
  3. Verify that there are no erroneous ACL or route entries in the work network switches/routers rendering those specific client machines incapable of accessing the sites.
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