This is part of the book "Introduction to Networking" at www.net-intro.com

Using the Internet seems pretty easy. We go to a web address and up comes a page. Or we go to our favorite social site and see pictures of our friends, families, and pets. But it takes a lot of complex software and hardware to make the Internet seem so simple. The design of the technologies that make today's Internet work started in the 1960s, and there were over 20 years of research into how to build internetworking technologies before the first "Internet" was built in the late 1980s by academics in a project called NSFNet. Since then, the research and development into improving network technologies has continued as networks have become far larger and faster and globally distributed with billions of computers.

This is part of the book "Introduction to Networking" at www.net-intro.com

To engineer and build a system as complex as the Internet, engineers try to break a single challenging problem into a set of smaller problems that can be solved independently and then put back together to solve the original large problem. The engineers who built the first Internets broke the overall problem into four basic sub-problems that could be worked on independently by different groups.  They gave these four areas of engineering the following names: (1) Link, (2) Internetwork, (3) Transport, and (4) Application.  We visualize these different areas as layers stacked on top of each other, with the Link layer on the bottom and the Application layer on the top.  The Link layer deals with the wired or wireless connection from your computer to the local area network and the Application layer is what we as end users interact with.   A web browser is one example of an application in this Internet architecture.

This is part of the book "Introduction to Networking" at www.net-intro.com

The lowest layer of our Internet Architecture is the Link layer. We call it the "lowest layer" because it is closest to the physical network media. Often the Link layer transmits data using a wire, a fiber optic cable, or a radio signal. A key element of the Link layer is that usually data can only be transmitted part of the way from the source computer to the destination computer. Wired Ethernet, WiFi, and the cellular phone network are examples of link layers that can transmit data about a kilometer. Fiber optic cables, particularly those under the oceans, can transmit data up to thousands of kilometers. Satellite links can also send data over long distances.

This is part of the book "Introduction to Networking" at www.net-intro.com

Now that we can move data across a single link, it's time to figure out how to move it across the country or around the world. To send data from your computer to any of a billion destinations, the data needs to move across multiple hops and across multiple networks.  When you travel from your home to a distant destination, you might walk from your home to a bus stop, take a train to the city, take another train to the airport, take a plane to a different airport, take a taxi into the city, then take a train to a smaller town, a bus to an even smaller town, and finally walk from the bus stop to your hotel. A packet also needs to take multiple forms of transportation to reach its destination. For a packet taking its "trip" to another country, the "walk", "bus", "train", and "plane" can be thought of as different link layers like WiFi, Ethernet, fiber optic, and satellite.

This is part of the book "Introduction to Networking" at www.net-intro.com

The Domain Name System lets you access websites by their domain name like (www.net-intro.com), so you don't have to keep a list of numeric Internet Protocol (IP) addresses like "212.78.1.25". IP address are determined by *where* your computer connects to the Internet. When you have a portable computer and you move from one location to another, you get a new IP address at each new location. Since no one connects to your portable computer, it does not matter if your IP address changes from time to time. But since so many people connect to a web server, it would be inconvenient if the server moved to a new location and needed to change its IP address.

This is part of the book "Introduction to Networking" at www.net-intro.com

The next layer up from the Internetworking layer is the Transport layer. A key element of the Internetworking layer is that it does not attempt to guarantee delivery of any particular packet.   The Internetworking layer is nearly perfect, but sometimes packets can be lost or misrouted. But users of the network want to reliably send entire files or messages across the Internet. A network is not much good to us if all it can do is send packets that are received most of the time. For the network to be useful, all of the packets need to be reassembled into the right order to reconstruct the message on the receiving system. The network must also deal with packets that arrive out of order or never arrive at all. The Transport layer is where we handle reliability and message reconstruction on the destination computer.  Just like the IP layer, the Transport layer adds a small amount of data to each packet to help solve the problems of packet reassembly and re-transmission.

This is part of the book "Introduction to Networking" at www.net-intro.com

We have been working from the bottom to the top of our four-layer TCP/IP network model and we are finally at the top.  The Application layer is where the networked software like web browsers, mail programs, video players, or networked video players operate. We as users interact with these applications and the applications interact with the network on our behalf.

This is part of the book "Introduction to Networking" at www.net-intro.com

In the early days of the Internet, networks were small and all of the routers were in secure locations. As long as each computer connected to the Internet protected itself from unwanted incoming connections, it was felt that there was no need to protect data from prying eyes while it was crossing the network.

So the Link, Internetwork, and Transport layers were focused on the efficient movement of data and solving the problems of a large-scale shared distributed network without worrying about the privacy of that data.

But as the use of the Internet grew rapidly in the late 1980s and literally exploded when the Web became mainstream in 1994, security and privacy of network traffic became very important problems to solve. When we began using the Internet to conduct commerce and credit cards and bank account numbers were being routinely sent across the network, securing data became essential. And when we started using wireless technologies like WiFi, security became necessary for even the simplest uses of the Internet.

This is part of the book "Introduction to Networking" at www.net-intro.com

So far we have spent all of our time describing the four-layer model used to design and implement the TCP/IP protocols and applications that make up the Internet. However, the TCP/IP  model is not the only model we can use to help us understand how networks work. The other model commonly used to make sense of network design is called the Open System Interconnection (OSI) model. While the TCP/IP model was designed and evolved as the TCP/IP protocols were developed, deployed, and changed, the OSI model was the result of a careful design process by many networking experts who worked to develop a general approach to network models.

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.