There is a Linux hardware based solution which works well for individual and SOHO users. The entire firewall program is maintained on the Floppy disk in the machine. Other hardware firewalls are located within routers. The programs in these screening routers screen packets on a criteria such as the type of protocol, the source address, and destination address fields of the protocol. My ISDN dial-up router contains such screening filtering capability.
Firewalls can be software based as well.
Software firewalls reside on the local machine and do the same functions as the hardware variety. When activitated, these programs filter incoming and outgoing data in accordance with predetermined filters that limit incoming and outgoing traffic. BlackICE Defender and ZoneAlarm are two such programs. Both set up filters which allow screening of incoming and outgoing IP and network traffic. Since most networks are now IP protocol based, this two-way filtering is necessary.