The Internet is a network of networks. It connects together different networks, and allows computers to communicate with other computers across these different networks. It works because of devices called routers. This is actually a word that's pronounced differently in British and US English. In the UK, we call them routers, but in the US, they'll call them routers. I will try to use the US pronunciation because it's more common internationally. A router connects together two networks, forwarding messages from one network to the other, and also translating between network protocols. A message sent on the Internet doesn't just go direct from the sender to the receiver. It's forwarded by a lot of routers across many networks before it gets to its destination. Most of us will connect to a local network in our home, our work, or a public space, like a library or an Internet cafe. These networks don't generally communicate directly with each other, but connect via big networks called Internet service providers, or ISPs. These networks can span whole countries, and are normally an evolution of the old wired telephone networks. National ISPs are connected to international ISPs, often called the Internet backbone. So if I were to send a message from my home in London to a learner in an Internet cafe in Nairobi, the message would pass from my home network to my UK ISP. And on to the Internet backbone, passing through many countries, and under the sea until it connects to a Kenyan ISP, and eventually with the Internet cafe's network. In fact, it might be even more complex, and I might send the message via Coursera, which would mean going through its network in California as well. To make all of this work, it isn't enough to have physical routers. We also need a protocol. Up to now, I've talked as if you only ever use one protocol at a time, but in fact, Internet communications use a lot of protocols at the same time. I might use the Wi-Fi protocol to communicate across my home network, but I also use another protocol to communicate across the Internet. This protocol is called the Internet Protocol, or IP. The Internet works because all computers and routers understand the same Internet Protocol, and so can communicate even though they may be on different networks and in different countries. You can think about protocols a bit like envelopes. I have an actual message I want to send, perhaps an email. I put that message into an envelope, which implements a protocol. With physical posts, the protocol is that I need to put on a name, an address, and a stamp. With those three things, the postal service will deliver my message. With Internet Protocol, I do something similar, putting my message into a virtual envelope called a packet, which includes an Internet address and other information. So my email to Nairobi would include the Internet address of the learner. The trouble is that my home network doesn't know how to send things to Nairobi, and uses Wi-Fi protocol, not Internet Protocol. So what happens is that I put my IP message into another packet that uses Wi-Fi protocol, and includes the address of my home router. The message is sent to the router, where it's taken out of the Wi-Fi envelope, but my router still doesn't know how to send things to Nairobi. All it can do is put my IP packet into another packet, this time using my ISP's protocol. That is how the Internet works. There is a message sent using Internet Protocol, but to be carried across actual network, this message needs to be wrapped up in envelopes, or packets, that use the protocols of each specific network. In fact, it's even more complex than that. The messages that are inside the IP envelope will often have their own protocols, depending on the application they use. For example, email messages use the sendmail protocol, and web pages use the HTTP protocol. Each of these protocols is wrapped up in an IP packet, which might itself be wrapped up in a Wi-Fi packet when being sent over Wi-Fi. There's even other protocols that I won't go into here, like TCP, which means that a typical message traveling around the Internet might use four or five protocols wrapped up in other protocols. To carry on with the envelope analogy, if you want a letter to arrive at its destination, you need to include an address. The same goes for the Internet. Every computer on the Internet has what is called an IP address. Knowing the IP address of a computer is what allows routers to know how to reach any other computer on the Internet. But hang on a sec, you might be thinking, doesn't an Internet address look more like this? That is a more user-friendly type of address that we're used to. It's called a URL, or uniform resource locator. URLs make it easy for humans to remember and type in Internet addresses, but they're more complex for routers to use. This middle part of a URL is called the domain name, and it tells you the name of the server where the website is stored on. This domain name is converted to an IP address, using an Internet service called a domain name service, or DNS. There's one last thing I have to mention before we finish this lesson on networks. Up to now, I've been talking as if I send messages from my computer to another person's computer, but it generally doesn't work that way. When I send an email, it first goes to an email server. This is a big computer somewhere on the Internet that handles a lot of people's emails. If I'm using my company email, the server might belong to my company, but if I'm using a public email service, like Gmail, the server would belong to Google or whoever runs the service. Similarly, my email server would send a message to the email server that my receiver of my message uses. It uses the second part of the email address of the receiver, the part after the @ sign, to work out which server it should send the message to. When I receive emails on my phone, it first has to contact the email server to download the new email that it's received for me. So most of the time, we're communicating with servers. The same goes for websites. I get a person's web page from a web server, not from an individual's computer. This has been a flying tour of how the Internet works. The Internet is a global network, made by connecting together different networks. It allows computers and people to communicate across the world, and is one of the most important innovations of our lifetime. I'm sure you use the Internet almost every day, like most computer and smartphone users. I hope what you've learnt in this lesson will help you understand it more, and also help you be a better Internet user.
