What is a router or firewall? Which do I need?
The short answer: All routers are firewalls.
A router is a device that allows an Internet connection to be shared by more than one computer. Without a router, a cable modem or DSL modem can only be connected to and used by a single computer. Typically, a router will have one connection to the Internet and four connections for computers, along with an optional wireless antenna.
Network Address Translation (NAT) is a feature that began being used in routers in 1998 and enabled inexpensive routers to provide complete firewall security. Prior to the creation of NAT, routers and firewalls were separate and expensive devices.
NAT automatically prevents all outside computers from accessing computers attached to the router while permitting computers attached to the router access to the Internet and e-mail.
Routers are available with Stateful Packet Inspection, which adds additional features to prevent software attacks against the router. However, the type of software attacks that an SPI router protects against (such as Denial-of-Service, SynFlood and Ping-of-Death) typically occur at Internet Service Providers rather than offices with private computer networks, so an SPI-equipped router is rarely, if ever, necessary for a home or office.