I'm posting this for anyone who encounters the issue I discovered.
Apparently, because Verizon FiOS uses PPPoE on their networks I had to adjust the MTU size from the default 1500 to 1492 on my ZyZEL ZyWALL USG 100 so anyone with FiOS service can get to my website hosted from my location.
I have both FiOS & Comcast ISP's and installed the USG 100 to replace a Cisco RV042G router. After the switch any of our employees who had FiOS service at their homes could not get to our CRM website. Anyone who had Comcast, RCN or other ISP service could get to the website without any problem. I was able to duplicate the problem with a laptop attached to the Verizon router on a separate static IP in our office.
After 2 weeks with FiOS Tech support, who told me nothing, I contacted ZyXel Tech support. We tested for another 4 days and finally found that decreasing the MTU size to 1492 solved the problem.
After digging into the Verizon FiOS Actiontech router, it's set with the MTU size of 1500.
If you use ZyXEL USG products (maybe others) and FiOS users can't get to a website hosted behind that device, try changing the MTU size. I'm guessing there are some that don't even know this problem exists.
PPPoE uses 8 octets for the header which reduces the remaining maximum size to 1492. If you search for "PPPoE verizon fios" you can find the Verizon website that allows you to select various states and shows if you might be on a PPPoE network.
This post could also be applicable to the ZyXEL forum.
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