Charlie,

I will guess you are behind a router at you home, if you are look at the settings. If you have not opened port 80 or any other port for that matter, they should all be closed. There is an exception with some manufactures that mistakenly left a port or two open causing a security risk.

So

When you make a request to a sever on a port, 80 or any other, the packets are addressed so the server knows where to send the requested packets back to. Your router will do something called NAT traversal to allow the traffic back through to the correct machine.

Lets say you set up a sever behind your router / firewall and you want it to receive request from the outside world, you will have to configure your router / firewall to pass that traffic through to the correct machine. Or place the home sever in the router's DMZ, but that is risky as far as security goes.

Most companies with any IT department worth anything will have every port closed to request from the outside. The one exception we commonly see is the use of a VPN tunnel.