If this Linux box will be connected directly to the Internet, an ISP may provide a default gateway to use.
The "Enable routing" option can be selected to enable this machine to serve as an Internet gateway for other computers on a local network.
A Linux gateway machine can share a dial-up connection, cable modem, DSL connection, etc.
In the example shown here, this Linux box will be using the local gateway 192.168.0.1. That machine has one network card attached to an ADSL modem and another network card attached to the network hub.
The Mac and Windows screenshots seen on previous pages show them also using 192.168.0.1 for their Internet access.