This is part of the book "Introduction to Networking" at www.net-intro.com
It has been said that building the Internet solved the world's most complex engineering problem to date. The design and engineering of the Internet started well over 50 years ago. It has been continuously improving and evolving over the past 50 years and will continue to evolve in the future.
The Internet now connects billions of computers using many thousands of routers and link-level connections. The Internet is so complex that it is never fully operational, The Internet is less about being "perfect" and more about adapting to problems, outages, errors, lost data, and many other unforeseen problems. The Internet is designed to be flexible and adapt to whatever problems are encountered.