eli5 Networking : Routers and ISPs

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I just recently found out that routers connect you to the internet, but then I found out that ISPs also connect you do the internet. Can someone explain to me what really happens behind the scenes? Why do we need ISPs if our routers can connect to the internet by themselves?

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> Why do we need ISPs if our routers can connect to the internet by themselves?

Your router connects your computers to each other, and to a computer owned by your ISP.

If you don’t have an ISP, the various computers and devices in your house can upload and download information to each other, but there’s no path for information to flow between your house and Reddit (or Google, or the tiny company that’s somehow still hosting this one obscure niche site from 2003).

Basically, someone has to own and maintain the backbone routers and switches. The big ISPs then pay for rights to use those backbone routers, and then sell that bandwidth to smaller ISPs who do the same. Eventually it filters down to you having to pay an ISP for rights to use the bandwidth they are renting from the next bigger company in the chain.

Why ISPs and not individuals? It’s because it takes specialized knowledge to program and protect these switches from both physical and cyberattacks. There are also things such as liability for the data being transmitted, guaranteed uptime requirements, and even just basic stuff like who owns what IP has to be handled by someone who can be trusted to handle it right and handle it consistently. There are also serious implications to national security if those backbone routers were compromised.

All of these costs and responsibilities combined are far more than the average person could ever hope to afford or do on their own. Hence, we have ISPs.

Can you elaborate on “routers connect you to the internet”? You can’t connect to public domains without a public IP address, which you would only get as a consumer from an ISP

Computer networking boils down to sending and receiving information from one computer to another.

Now think of this like a delivery problem. Imagine you need to deliver something (the information) with your car from your house (your computer) to somewhere else (another computer). To oversimplify, your driveway is the router/modem, the street you live on is a small part of your ISP. Both of them together connect you to a much more sophisticated road network which is the internet.

If you have an ISP but no router/modem, then you have a house that is isolated by itself with no road access. If you have a router/modem but no ISP, then you have a driveway that isn’t connected to any road, ie it’s useless pavement. You need both of them to access the bigger network.

It’s very similar (conceptually) to how the landline phone system works… You have a phone in your house, it’s connected to a local exchange, that’s connected to a regional exchange and that’s connected to an international exchange. Depending on where you’re calling, you’re phone will be connected along different paths.

If you switch perspective to that international exchange the system can be viewed as a bunch of trees (like a family tree) branching out from that central point.

Your router, is like a landline phone, connected to your ISP, which is a local provider, and they’re connected to another provider (usually referred to as a backbone providers). If your router wasn’t connected to your ISP, your computer’s could talk to eachother, but couldn’t talk outside your home, and if your ISP wasn’t connected to a backbone provider (as sometimes happens) you’d be able to connect to some sites (like your ISPs own website) but not all sites, depending on where exactly the break is – similar to how an international exchange problem might stop you calling the UK, but you could still call Canada or Mexico