At the heart of the ICS network segmentation are the firewalls. They control what traffic is allowed into and out of the IDMZ and run inspection on the allowed traffic. They look for abnormal protocol behavior, search for patterns of compromise, and verify traffic signatures against known malware and exploit traffic. Let's look at the steps involved in setting up a pair of Cisco ASA firewalls that are used in the IDMZ. Note that these are the shortened and simplified steps taken from the publicly available design and configuration manual found at https://www.cisco.com/c/en/us/td/docs/solutions/Verticals/CPwE/3-5-1/IDMZ/DIG/CPwE_IDMZ_CVD.html:
