Double NAT

A problem I encountered was trying to configure port forwarding on a simple office network.
I had failed to realize though that both the cable modem and the all-in-one consumer router I had both had firewalls of their own.
One thing to always check for in these circumstances is that the modem is set to bridge mode, allowing the router to take up these jobs.
You can use an online port scan to see if your port forwarding is successful. Most of them are nice enough to provide your current IP to test on. Note in large infrastructure, this could be problematic! Check beforehand.