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A Firewall is a hardware or software device which is configured to permit, deny, or proxy data through a computer network which has different levels of trust. A firewall is a literal wall of fire, a barrier used to inhibit movement of enemy forces.
Phong and Mouse came up with the idea to enclose Giedi Prime sector in an inverted firewall. The firewall while brilliantly effective is not a self-sustaining program and has some technical limitations. It relied on energy being redirected from the Principal Office shields as both cannot be used simultaneously. The Principal Office shields had to be deactivated in order to power the senders which consequently leaves the city completely defenseless should the firewall somehow shutdown.
Hue Branch and the Tech Boys devised emitters that could create the required firewall, they were scattered by Mouse, and imprisoned Megabyte and Hexadecimal within G-Prime so that he could not attack the rest of Mainframe. Enzo Matrix activated the firewall via a command to Glitch.
With Giedi Prime now imprisoned, Megabyte had no other choice but to rely on Hexadecimal's viral energies to create a rift in the firewall. Hexadecimal was forced through countless painful shocks to focus her powers into a point, thus creating a small opening that allowed Megabyte's ABC army to pass through two-by-two. The Principal Office managed to put out sufficient firepower to repeal the attack. The ABC's turned back, but Megabyte ordered the rift closed. When Hexadecimal stopped focusing on the wall the rift closed and the ABC's collided with the wall. After most of the viral ABCs were wiped out, Megabyte ordered Herr Doktor to begin working on their own Firewall Disruptor to replace Hex.
Unfortunately the firewall was unable to contain the viruses for long. Not long after Mainframe suffered the loss of their young Guardian Enzo, Hexadecimal escaped from Megabyte's prison and destroyed the Silicon Tor. She then overloaded every firewall emitter in a single shot, appearing as to take very little effort.
During the Net War, Mouse used a firewall to re-seal Mainframe from the rest of the Net so that Daemon couldn't get into the system and infect it. The Guardians were ordered to try and breach the firewall using a decoder computer. This attempt failed as the decoder was unable to analyze the complex code created by Mouse.
Mouse was infected by Daemon soon afterward during a failed assasination attempt. Dot Matrix had her staff try and change the security code to the firewall. When they failed, Dot came up with the idea of creating a second firewall. They simply planted senders outside the rim of the first firewall and activating them. The second firewall was not as densely coded and was easily hacked by Mouse when she arrived. It did buy Mainframe precious time to devise a strategy against Daemon.
The CPUs had managed to use Firewalls more intuitively and compact them into much smaller grids for use within the Principal Office's holding cells. One such Firewall was imprisoning Megabyte's alias. (Crouching Binome, Hidden Virus)