In computing, a firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. A firewall typically establishes a barrier between a trusted internal network and untrusted external network, such as the Internet.

Two variants of firewalls can be used in City Cloud

  1. Security Groups (OpenStack)
  2. Dedicated Firewalls

Do we need to mention NAT here?
This page is rather thin, but it is only a brief introduction, more details are explained in the underlaying pages.