Internet Network Example
Here's an example. Imagine that Company A is a small firm that has its office network set up with a server and a networked printer. Imagine that Company B is a corporate ISP. Company B builds or leases office space in major cities to store its servers and routing equipment. Company B is so large that it runs its own fiber optic lines between its buildings so that they are all interconnected.
In this arrangement, all of Company A's customers can talk to each other, and all of Company B's customers can talk to each other, but the two companies' networks aren't linked. Both companies can communicate internally, but neither can communicate with the other. Therefore, Company A and Company B both agree to connect to internet access points, or IXPs, in various cities. The two firms' networks can now connect to each other and other organizations using the internet.
This example shows how two companies' networks talk to each other, but these two businesses are just a close-up example showing how their two networks join the vast internet. To get a bird's-eye view of what these interconnected networks look like, take a look at Barrett Lyon's Opte Project, which endeavors to create an evolving map of internet pipelines.