This is an old one but it reared its ugly head recently.
Client has no filtering on their router at all. Certain sites wouldn’t open, for example, banking sites. I did a little research and I discovered that the MTU was set to high. What’s an MTU? It’s the maximum size of information you can transmit. Anyway, it doesn’t really matter, what’s more important is how to fix it.
1. Open your command prompt
2. Type in the following command: ping -f -l 1492 www.yahoo.com (1492 is the max that most ISPs allow for)
3. Odds are, you will get a fragmented message
4. Subtract 8 from 1492 and ping again (e.g. ping -f -l 1484 www.yahoo.com)
5. Keep doing it until you get a reply (the value below of 1300 is strictly for illustration purposes)
6. Take this value and adjust your router to reflect this value