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)?