One way to protect a network is to physically isolate it from unsecured networks. Typically, this means that the air-gapped network is not linked to the internet. Given that many of the the attack vectors come from the Internet, this is seen as a much more robust approach. It is used in critical infrastructure environments such as nuclear power stations.
But, these networks are not immune to attach as testified by the attack on Iran’s nuclear infrastructure by the Stuxnet Worm in 2010. The attack is believed to have been undertaken by American and Israeli intelligence services – the whistleblower Edward Snowden confirmed this in 2013. The Stuxnet Worm was introduced to the Iranian air gapped network via a USB Flash Drive.
Not connected to a public network.