The term Gateway is used in networking to describe the “Gate” to the Internet. The Gateway controls traffic that travels from the inside network to the Internet and provides security from traffic that wants to enter the inside network from the Internet. The Gateway is often the router that home owners or businesses use to connect to the Internet. Because some networks may have a number of connections to several other internal networks the term “Default Gateway” is used to signify the route for traffic which is the first choice.
The default gateway is the machine IP number that you need to access to get to the rest of the network or the Internet. Gateways connect two different kinds of networks, like your work network and the Internet. You will need to know the specific address of your gateway to connect to the Internet.
In the image above the Gateway IP Address would be 192.168.5.222, using the example addresses from above. This gateway could be a router that would have two network connections, one to the internal network and one to the Internet. The IP Address of the workstations and web server would need to be addresses on the 192.168.5.0 network to connect to the gateway. Notice that the workstations and server all connect to a hub/switch before they connect to the gateway since the gateway will probably have only one connection.
When you add a computer to your home network that needs access to the Internet, one of the required settings is the Gateway. Your computer needs to know how to get to the Internet, thus the Gateway. You then need to supply the IP Address of your Gateway so that computer has access.