Firewalls are your one defense against enemy players. These firewalls are used to protect the ports from being hacked. Firewalls, and their stats, can be accessed through the 'My Computer' app/window. There are three designated ports that each player has access to, ports A, B, and C. Each port can have up to 30 charges and each has the ability to chose the upgrades you want.
Once any enemy player has chosen you as their target, they are then prompted to chose any of the three ports. Early on it may be ideal to use 'Charge x 5' to keep your ports healthy as the firewall gets hacked when the charge drops down to zero. It's also ideal to upgrade strength early on as this prevents players from easily gaining access to your BT coins. Currently the charge can be upgraded to a maximum of 30.
In addition to the 'Charge x 5' option, the firewall charges itself. The rate at which the firewall regenerates itself can be increased by using the 'Increase Regen' Upgrade. It can be upgraded up to 10 times.
The 'Upgrade Strength' Upgrade increases the difficulty of hacking your Firewall. Upgrading this will give the enemy much difficult commands to enter into CDM. This can be upgraded up to 4 times.