Do a netstat -an
and check where the web-server is listening (it may be only bound to 127.0.0.1), you want it to be listening on 0.0.0.0 for all interfaces.
Then check if you can ping the machine from another machine on the local network, if not, and you are happy you have physical connectivity, ie., you can ping other machines (not one of your machines IP addresses) on your local network as you describe it, then you have a firewall miss-config.
Simplest thing to do here to check, is simply to turn off he windows fireall (only if you are on a trusted private network). If the problem goes away, checkout the firewall rules and make sure you are allowing http (tcp/80) and https (tcp/443) through it to the IIS process.