Nitpicking here, but unless there are multiple sites, that’s just a LAN (local area network), and even across multiple sites without certain other bits and pieces, it’s still “only” a WAN (wide area network).
Admittedly, the latter can be called an internet (lowercase i important). The LAN would be an intranet.
To replace the Internet (capital-i and all) you’d need, at the very least, a DNS server of some sort, if not reimplementation of any tools that assume IANA and ICANN are at the top of the hierarchy (or successfully lying to them about where those roots are on the network).
To head off the question of where VPNs fit into all of this: Any or all of this could be implemented over a VPN that uses the existing capital-I Internet to transfer data, at the expense of some bandwidth.
Some care would be needed to not let data leak onto the real Internet at any of the end points, but it could be done. This is basically what TOR does for the Dark Web.
Nitpicking here, but unless there are multiple sites, that’s just a LAN (local area network), and even across multiple sites without certain other bits and pieces, it’s still “only” a WAN (wide area network).
Admittedly, the latter can be called an internet (lowercase i important). The LAN would be an intranet.
To replace the Internet (capital-i and all) you’d need, at the very least, a DNS server of some sort, if not reimplementation of any tools that assume IANA and ICANN are at the top of the hierarchy (or successfully lying to them about where those roots are on the network).
To head off the question of where VPNs fit into all of this: Any or all of this could be implemented over a VPN that uses the existing capital-I Internet to transfer data, at the expense of some bandwidth.
Some care would be needed to not let data leak onto the real Internet at any of the end points, but it could be done. This is basically what TOR does for the Dark Web.