Firewalls are available
with a variety of capabilities, ranging from simple devices,
which block communication based on source or destination
addresses to powerful devices, which are able to inspect the
contents of communication and dynamically decide whether
information should be passed on or blocked. At the minimum,
a firewall should be placed between the industrial network
and any external network to which it connects. However, a single firewall may often be inadequate, depending on the level
of access that is required.