Subnet

6 routers. What the? Since I dont know how you plan on networking these 5 business, I assume they are very small businesses that have not been networked as of yet. First off, I would recommend each office having their own switch. You will network each office as a star topology, all directly connected to the office switch. (Each computer is directly connected to a port on the switch, while leaving 1 port open). Once all the offices are wired and are connected to their own switch, you will then connect each open port on the 5 switches to the router (which will need to have a built in switch in order to communicate with all the other switches) Or, if all the switches are located in one spot, you can daisy chain the switches and then connect to the lan port of the router. The router will have to have built in dhcp or you will have to set up static addresses. You will also have to have the router clone the Mac address of the computer that was registered with the cable service provider, in order for it to send proper configurations to the clients (as well as working at all). You will have to use NAT (Network Address Translation) within the router in order for this to work. Providing more info would help. Good luck

The best way I would do it would put like a sonicwall (www.sonicwall.com) at everyone's location befor a switch that goes to the cable modem. What way they can see out but nothing no one else can see in. If anyone wants to see in, they would have to creat a VPN that would accept that connection. Being certified in installing firewalls, its the best way to go that will protect each business from seeing each other.