The simple thing to check when network traffic is blocked

If inbound or outbound traffic is blocked, it can be very frustrating figuring out what's wrong. However, often the actual problem is very simple. One of the most common problems is forgetting to open the correct port. For example, suppose you're Point-to-Point Tunneling Protocol (PPTP) for VPN. This protocol uses TCP port 1723 to establish a connection. If that's closed, your VPN isn't going to go anywhere. So next time traffic is mysteriously blocked, check the ports.