Firewalls are your one defense against enemy players. Theses 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. It's also ideal to upgrade strength early on as this prevents players from easily gaining access to your BT coins.