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The ISP is in the same building, they provide the internet from an RJ45 cable through a hole in the adjacent wall, instead of the conventional coaxial or RJ11 modem. Right now there is an ethernet switch directly connected to the RJ45 cable, and that gives internet and LAN to all the devices connected to the switch, which means that the RJ45 comes from a router behind the wall (on ISP's side).

I researched that if I connect the RJ45 to the WAN port of a router of mine, then the ISP couldn't reach inside of my network, but my network could still reach computers eventually available inside the ISP's network, only if I know the IP adresses of those computers, I assume this is below the conventional way in terms of security, or is less appropriate.

Besides of having a router of mine connected from its WAN port to the ISP's RJ45, what else is necessary to have a separate network, i.e. equivalent to those that are connected to an RJ11/coaxial/modem)?

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The medium they use for the connection has nothing to do with security. You could equally put all your computers on the public internet by connecting it to a DSL/coax modem (not a router).

In this case it looks like due to the fact that every computer has an Ethernet card the ISP decided not to provide a router, and as a result people are putting their machines directly onto the Internet. Nothing prevents you from putting your own router/firewall in the middle if you need the extra security.

As far as reaching other computers by IP, this is exactly how the internet is designed to work. You may have had the impression that it isn't the case due to an awful hack called NAT which hides multiple computers behind one single IP - the router has the public IP and everyone else behind it uses a local, private IP, and to receive incoming connections you had to tell the router to forward particular ports to the local IP. In this case since people aren't using routers no NAT is happening and every device has a publicly routable address.

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