is related to that resource. It is possible to have many levels of security within a company or system. A good idea is to avoid using more security than necessary to avoid unnecessary inconveniences. What is experienced as complications can lead to confusion and less efficiency. However, one of the greatest threats in all computer systems is the company's own staff. Most of the investigations done in this area show that the staff inside the company is one of the most frequent intruders in computer systems. 6.9 INTEGRITY BY ACCESS CONTROL - FIREWALLS 6.9.1 WHAT IS A FIREWALL? In general, a firewall is a node (computer and/or router) often located at the border between the public Internet and servers or a private intranet. See Figure 6.18. The purpose of a Public Internet Firewall Figure 6.18 Location of a firewall Understanding Changing Telecommunications - Building a Successful Telecom Business firewall is to block access to the intranet from the public Internet. People outside the intranet must not be able to access information inside the intranet or perform actions such as destroying information. At the heart of all firewalls is a packet filtering or screening mechanism, based on an interpretation of packet type. The simple firewall cannot check the application level, but identifies IP, TCP, UDP and ICMP. The filtering is controlled by a table. By reading the TCP header, especially the TCP port, it is also possible to determine the type of application. Port 80 in the header means web traffic. Thus the firewall can for example decide that web traffic can pass whereas FTP traffic is blocked. The firewall translates the IP addresses by means of network address translation (NAT). With NAT the internal IP addresses in a LAN or Intranet are not visible from the out- side. This increases the security for the enterprise and reduces the necessary number of IP addresses. 6.9.2 TERMINOLOGY FOR PROTECTED AREAS Protected Network (Inside) All internal systems are placed on the protected network and cannot be reached from any