The more expensive routers are modular, letting you add interface cards for Ethernet, token-ring, ATM, or FDDI ports. This sends the caller's address to the receiving router through the ISDN D channel, letting the device apply any security rules set up for that user. Routers vary in how they handle watchdog traffic sent out by the LAN to see that services are still available. Such traffic can disable the capability of the router to take advantage of dial on demand and automatic disconnection. It can also prevent the router from supporting bandwidth on demand.
Some routers spoof the watchdog traffic.
Others, which eliminate the watchdog traffic, require you to manually set up the routers to define watchdog traffic. If several users share a router, some models let them share an IP address as well. This can save money and increase security. Because all users behind the router have virtual addresses, which are invisible on th e Internet, the router acts as a firewall. How-ever, because the router has to find the virtual IP address, you can expect a performance degradation of 5 percent to 15 percent.