Add a 2nd eth port to your computer, attach a switch/router that the offline devices connect to, using same class C network the 2nd port uses (different from th

Add a 2nd eth port to your computer, attach a switch/router that the offline devices connect to, using same class C network the 2nd port uses (different from the one on 1st port that connects to broadband router/network). Unless you bridge your ports, the other devices can talk to you and each other, but not the internet. Semi-educated guess :)