if you've already set up your Tomcat on a server machine with an internal IP (e.g. 10.x.x.x or 192.168.x.x), you can configure port forwarding on your office's router so that all accesses to port 80 of the static IP exposed by the router will be forwarded to your machine.
if you want to expose your machine fully to the Internet, just assign that static IP to your server where you installed Tomcat. then configure your router properly, in case the router and the static IP you assigned to the Tomcat server are different.