Behind every website, Universal Resource Locator
(URL), and computer or other device connected to
the Internet is a number that uniquely identifies
that device. This unique identifier is called an IP
address. These addresses are the key components
of the routing schemes used over the Internet. For
example, if you are downloading a data sheet from
www.cisco.com to your computer, the header of
the packets comprising the document includes both
the host address (in this case, the IP address of
Cisco’s public server) and the destination address
(your PC).