A firewall was a software appliance that controlled access to, and protected, a networked computer system.
CTU Los Angeles had a firewall that protected its systems from external hackers. During Day 2, when Michelle Dessler worked from offsite, Tony Almeida gave her a Huffman coded data card containing software allowing her to break through the firewall. During Day 3, Nina Myers activated a worm at CTU that threatened to eat through the firewall, exposing information on their agents to the entire world. However, Chloe O'Brian stopped the worm before that happened. ("Day 2: 5:00am-6:00am", "Day 3: 1:00am-2:00am")
On Day 4, when Marianne Taylor arrived at CTU, Curtis Manning asked Chloe to build a firewall around her station to make sure she didn't stray outside her assigned area of work. ("Day 4: 9:00am-10:00am")
Edgar works to stop the firewall intrusion
The Nuclear power plants of America had firewalls to protect their computer systems, but Habib Marwan used the Dobson Override to subvert them and cause the plants to meltdown. Edgar Stiles managed to counteract his actions, stopping all but one from melting down. ("Day 4: 1:00pm-2:00pm")
Lynn McGill set up an internal firewall at CTU to monitor everyone's activity, and when he was relieved of duty it was lifted with Curtis's level one access code. Later that day, Chloe O'Brian had to hack through the State Department's firewall to access the passenger list of Flight 520. ("Day 5: 4:00pm-5:00pm", "1:00am-2:00am")
Joseph Malina had a Phoenix Shield firewall protecting his computer systems. Christopher Henderson attempted to get him to lift the firewall, but CTU intervened too early and Chloe had to hack through it to access his files on the Natalia submarine. ("Day 5: 4:00am-5:00am")
At some point before Day 6, Nadia Yassir left the Jarvis firewall unsecured, but Chloe O'Brian managed to mitigate her actions and stop her from being fired. On Day 6, Mark Hauser had his brother break through the IPC firewall to provide Dmitri Gredenko with the details of Edgemont Nuclear Power Plant. ("Day 6: 6:00am-7:00am", "8:00pm-9:00pm")
The CIP device
Homeland Security designed the CIP firewall project, designed to protect all national infrastructure. However, a device was created by Michael Latham allowing Iké Dubaku to break through the firewall and cause several terror attacks during Day 7. ("Day 7: 8:00am-9:00am")
When Jack Bauer called Meredith Reed on Day 8, the ECHELON system picked up their call and Jason Pillar downloaded the entire conversation to a segregated server surrounded by firewalls to stop CTU from viewing it. Later that day, to stop Jack from distributing the recording implicating Yuri Suvarov in Omar Hassan's death, the NSA put up firewalls to stop it reaching the press. ("Day 8: 12:00pm-1:00pm", "3:00pm-4:00pm")
Margot Al-Harazi had firewalls inside of firewalls protecting her computer systems as she used the Defense override module to control U.S. drones. The device had the capability to break through most defense system firewalls, which Cheng Zhi used to sink the Shenyang and attempt to start a war between the U.S. and China. ("Day 9: 6:00pm-7:00pm", "7:00pm-8:00pm")
