Firewalls protect your computer from unauthorized access when you're connected to the Internet. Both hardware and software firewalls examine connection requests and, by applying a set of configurable rules, deny or allow the connection. The hardware required for a firewall is available as a dedicated unit; however, a reliable secure firewall can be created from basic PC hardware.
Chassis
Dedicated firewall chassis are designed to fit inside a computer rack, but you can use a spare PC case to house your firewall's components. The case must be compatible with the chosen motherboard and have enough room for the network interface cards.
Motherboard
The motherboard must fit the chassis, have at least two slots for the network cards and have an on-board video card. Using high-specification PCI (peripheral component interconnect) video card adapters is not recommended, as these produce more heat and the basic functionality required to operate the firewall doesn't warrant them. The motherboard should run at 133 Mhtz at minimum.
Network Cards
A firewall requires a minimum of two network cards. One card is used on the internal side of the network that your computer is connected to; the other is used on the unsecured external side of the network that connects to the Internet. Cards with a 100 Mbit/s throughput rate are acceptable for use in a firewall. Although not essential, it's advisable to use identical cards for ease of setup.
Memory
The minimum requirement for memory is 512MB for a firewall performing basic port blocking. Ports provide a communications "tunnel," and access to them is controlled by the firewall. If you want your firewall to perform more advanced functions, such as filtering the content of websites, more memory should be added.
Hard Disk
Firewall hard disks can be used to store logs that record intrusion attempts and network usage. They also run the chosen operating system and firewall software. One 40GB hard disk is adequate for a basic firewall; however, using two hard disks provides an extra level of fault tolerance if one should fail. Both disks should be set up as a mirror (identical data is recorded on both disks simultaneously): Should one fail, the other can be used to restore the data and reduce downtime.
Cooling
Your firewall needs to be on at all times while you're connected to the Internet. It's normal practice, in commercial settings, for a firewall to be left on 24 hours a day, seven days a week. Providing adequate cooling for the internal components is vital to avoid hardware failures. An additional cooling fan should be added to complement the existing cooling system within the chassis.
