Internet connection problems on small office network

Question

We have a small office network with an ADSL Internet connection connected to our network switch.

All 5 of our machines were running XP and we had no issues with our Internet connection. We upgraded 3 machines to Windows 7 and at the same time our ISP was bought out by another company. We are still using the same network switch and ADSL.

From then onwards we have been experiencing the following problem:
The Router shows both DSL and LAN link lights yet we cannot connect to webpages or download mail. We have to refresh a dozen times then it loads a page and then 5 minutes later same issue.

We have tried a new router, had our ISP try everything. Had our telephone line tested. But still this problem occurs.

Are we missing anything?

Answer 1

Well, the first thing you need to do is run a traceroute to find out where the problem lies.

Open a Command Prompt window and type tracert 74.125.230.148 (that's google.com by the way)

If you see something like this:

then you know the low-level networking is working OK and it's something specific to web browsing. If however you see this:

then you know that you can reach the router (the first entry in the list) but nothing further, so it's the connection from the router to the internet that is the issue.

If even the first entry is all * then you aren't communicating with the router at all and the problem lies within your network.

Answer 2

It's possible there's a Path MTU Discovery black hole somewhere at your ISP. Try dialing down the MTU on one of your machines from 1500 down to 1400 or even 1000, and see if the problem goes away for that machine. If it does, then try progressively higher MTUs until you find the highest value that works.

If your DSL ISP uses PPPoE, your MTU probably needs to be 1492 or less.