4-Port ADSL Router User Manual

Configuring IP Routes
86
In comparison, when your computer initiates communication over the
Internet, such as viewing a web page connecting to a web server,
the data it sends out includes the IP address of the destination
computer (the phone number). All your outgoing requests first go
to the same router at your ISP (the first switchboard). That router
looks at the network ID portion of the destination address (the area
code) and determines which next router to send the request to.
After several such passes, the request arrives at a router for the
destination network, which then uses the host ID portion of the
destination IP address (the local phone number) to route the request
to the appropriate computer. (The network ID and host ID portions
of IP addresses are explained in IP Addresses, Network Masks,
and Subnets.)
With both the telephone and the computer, all transactions are
initially sent to the same switchboard or router, which serves as a
gateway to other higher- or lower-level devices. No single device
knows at the outset the eventual path the data will take, but each
uses a specific part of the destination address/phone number to
make a decision about which device to connect to next.
Hops and gateways
Each time Internet data are passed from one Internet address
to another, it is said to take a hop. A hop can be a handoff to
a different port on the same device, to a different device on the
same network, or to a device on an entirely different network.
When a hop passes data from one type of network to another, it
uses a gateway. A gateway is an IP address that provides initial
access to a network, just as a switchboard serves as a gateway
to a specific set of phone numbers. For example, when a computer
on your LAN requests access to a company's web site, your ISP
serves as a gateway to the Internet. As your request reaches its
destination, another gateway provides access to the company's
web servers.