The default gateway is of such import to Internet communication that it gets it own chunk. It's an optional setting if you only want to communicate with the computer next to you, but it's required if you want to surf the Internet. In other words, it's required.
A default gateway is the IP address of the router. What's a router? It's a device with two (or more) network cards, each one on a separate network. It represents a physical means of getting information from one network to another, as shown in Figure 9-3.
Figure 9-3. The role of the default gateway.
In the case of the very simple router depicted in Figure 9-3, one interface is connected to the LAN, and one is connected to the WAN. The interface that ends with .1 is also the means by which computer A communicates with any and all remote networks. Computer A wants information from ebay.com? It sends requests through the default gateway. Help and Support from a Microsoft Web Forum? Again, the packet is sent to the default gateway, which helps forward the packet to its destination network. In fact, computers that live on the other side of routers are said to be on remote networks.
If there's a default gateway in the mail system, it's like your nearest mailbox. If you have a letter that will be delivered next door, you might just walk the thing over and save a stamp. But if it's going across country, all you have to do it is drop it off at the mailbox. It gets routed to the intended recipient without you having to give another moment's thought.