và cố gắng truy cập trái phép từ các địa điểm đáng tin cậy. Một IDS được xem là một hình thức của một kiểm soát an ninh kỹ thuật trinh thám. Một IDS có thể chủ động theo dõi các hoạt động đáng ngờ, kiểm tra nội dung các bản ghi kiểm toán, gửi thông báo cho các quản trị viên khi sự kiện cụ thể được phát hiện, | 104 Chapter 4 Communications Security and Countermeasures to the NAT server s. This change is recorded in the NAT mapping database along with the destination address. Once a reply is received from the Internet server NAT matches the reply s source address to an address stored in its mapping database and then uses the linked client address to redirect the response packet to its intended destination. This process is known as stateful NAT because it maintains information about the communication sessions between clients and external systems. NAT can operate on a one-to-one basis with only a single internal client able to communicate over one of its leased public IP addresses at a time. This type of configuration can result in a bottleneck if more clients attempt Internet access than there are public IP addresses. For example if there are only five leased public IP addresses the sixth client must wait until an address is released before its communications can be transmitted out over the Internet. Other forms of NAT employ multiplexing techniques in which port numbers are used to allow the traffic from multiple internal clients to be managed on a single leased public IP address. Switching Technologies When two systems individual computers or LANs are connected over multiple intermediary networks the task of transmitting data packets from one to the other is a complex process. To simplify this task switching technologies were developed. The first switching technology is circuit switching. Circuit Switching Circuit switching was originally developed to manage telephone calls over the public switched telephone network. In circuit switching a dedicated physical pathway is created between the two communicating parties. Once a call is established the links between the two parties remain the same throughout the conversation. This provides for fixed or known transmission times uniform level of quality and little or no loss of signal or communication interruptions. .
