I think you wonder whether your PC can be connected directly to the internet. Yes, like every server, router, firewall, technically you can. You would need to set up your connection completely manually, as there is no one doing that for you.

However, connecting millions of people is an epic task in itself. You would need at least as many public IP addresses as consumers have devices, and find a way to drag ethernet cables over enormous distances… while these won’t exceed about iirc 120m until signal loss is killing the connection. Then you would need all those consumers to have intricate network knowledge too.

Hence why we’re modems that can use long distance connections (like DSL over phone lines), that can arrange internal network connectivity without you having any knowledge, and apply internal IP space instead of public. Also your ISP can apply efficient internal routing and even internal IP space to save on rare and expensive public space.

There is much more to this, but I wanted to keep it ELI5. HTH!

There's no modem needed, actually. All of that can be done in software, and you can configure a desktop as a PPPoE client (that's the protocol your router uses to log into your ISP's network and receive internet connectivity). Obviously, you'd need to configure that PC as a router for other computers to also share the connection, and running a typical interactive system 24/7 as a router is hideously inefficient in terms of power use.

Most routers are actually small PCs, just optimized for the specific purpose of connecting to the internet over a phone or TV line, providing a separate internal network in your home and bridging the two. But you could do that with any regular PC with the right hardware.

Your ISP could also just provide you with a standard network connection, so you don't have to use a modem to transmit data over a line that was originally built for a different purpose. This is not very common since most houses still predate the internet, but it might become the norm at some point.

The main benefit of having a separate network is that devices in it are not directly visible and accessible from outside. All others see is your router, and they can only access your devices if you establish the connection first or manually forward ports. You can also structure the local network however you like (e.g. assign IPs and domain names, create subnets), without being restricted by or affecting the outside world.

yes, we can.

but it's a niche market so you have to diy

Thanks for all the responses. V interesting.

The other day while shopping for a new modem-router, we kept finding good deals only to realise that they were only routers. No fibre-to-the-house for us yet so dems no good. Don’t want to chase around finding a separate modem too. Got a TP-Link Deco in the end. It seemed the easiest / best thing available to the physical Saturday consumer in need.

If your ISP uses an Ethernet cable into your premises, then you most likely can do exactly that. Either you’ll get an IP from the ISP’s DHCP server or you’ll have to configure a PPPoE client with some login details before the ISP issues you an IP and connectivity.

To do that to every device on the globe you’ll need a much larger address space to IPv6 is your only choice.

That said, be mindful of the security that a router provides. Connecting your PC this way means you desperately need to keep it up to date and running restricted firewall.

Some ISPs run a coaxial cable into your house, though. Most of these use (Euro)DOCSIS and finding an interface card for a regular PC seems impossible.

All in all, it’s possible but it depends on the cabling standards and the protocols spoken. Don’t forget, at the end of the day, your modem is just a computer running Linux, most likely.

Check out OPNsense or OpenWRT if you’re interested in using a regular computer for routing. Alternatively just use Linux but it’s a lot more work.

That's actually more or less how it used to work. You'd plug your computer directly to the internet and connect that way. You'd have to dial into the internet. Constantly being online was a novelty. Especially when you paid by the minute. But then people started having more than one PC and thus home routers became a thing.