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The Internet as a Large WAN

The Internet is an amazing cultural phenomenon. Most of us use it every day. We post messages
on social media sites, we search for information using a search engine like Google, and
we send emails. We use apps on our phones to pull down information, like weather reports,
maps, and movie reviews. We use the Internet to purchase physical products and to buy and
download digital products like music and videos. The Internet has created completely new
things to do and changed the old ways of living life compared to a generation ago.

However, if you instead focus on the networking technology that creates the Internet, the
Internet is simply one huge TCP/IP network. In fact, the name “Internet” comes from the core
network layer protocol: Internet Protocol. The Internet includes many LANs, and because the
Internet spans the globe, it of course needs WAN links to connect different sites.

As a network of networks, the Internet is actually owned by countless companies and
people. The Internet includes most every enterprise TCP/IP network and a huge number of
home-based networks, as well as a huge number of individuals from their phones and other
wireless devices, as shown in Figure 3-12.

The middle of the Internet, called the Internet core, exists as LANs and WANs owned and
operated by Internet service providers (ISP). (Figure 3-12 shows the Internet core as a cloud,
because network diagrams show a cloud when hiding the details of a part of the network.)
ISPs cooperate to create a mesh of links between each other in the Internet core, so that
no matter through which ISP a particular company or person connects, some path exists to
every device.
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Figure 3-12 Internet with Enterprise, Home, and Phone Subscribers
Figure 3-13 shows a slightly different version of Figure 3-12, in this case showing the concept
of the Internet core: ISP networks that connect to both their customers as well as each
other, so that IP packets can flow from every customer of every ISP to every other customer
of every other ISP.
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Figure 3-13 Internet Core with Multiple ISPs and Telcos

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