Once your company is on the Internet, it is, of course, open to potential attack. An important first line of defense is a firewall, which prevents unauthorised access to your company's systems from external networks, and also prevents internal users from accessing areas of the network that they are not authorised to access. A firewall is a security control point between two networks. Apart from protecting your internal network from the Internet, there is often good reason to protect internal networks from each other.
Some firewalls can even protect subnets and individual machines.
However, it is important to remember that a firewall cannot protect your organisation from internal abuse by authorised users. It must also be remembered that installing a firewall is only part of the solution. The firewall acts as a gateway between your company's network and the Internet. In this way, a router can be used as a firewall, by using filters as an access-control device to decide which packets are allowed through and to which destinations. The firewall should also log access requests from one network to another - both those which are allowed through, and those which are denied.