There are numerous software available that allows an organization to restrict access to its intranet or part of its intranet. Firewalls provides security to the network using userids, passwords, encryption techniques, etc. An organization must decide who controls and maintains the information presented and who has what level of access to what data.
Most organizations allow internal users such as staff full access without further authentication.
One of the points of using an intranet is to allow staff to exploit the many resources on the Internet. It is recommended that users be divided into common groups and subgroups according to the level of access given to them. To be safe, it is usually recommended that unless explicitly allowed, access and services should be denied. The Rotherwick Firewall Resource - Includes great links to more security and firewall info.