to private addresses as required. Needless to say, dynamic NAT is by far the most common, as it is automatic and requires no intervention or maintenance. Port Address Translation One complication arises if the private network's gateway has only a single public IP address available to assign, or if more computers in a private network try to connect than there are IP addresses available to the gateway. This will often be the case for a small organisation with a single Internet connection to an ISP. In this case, it would seem that only one computer within the private network would be able to connect to the Internet at a time. Port Address Translation (PAT) overcomes this limitation by mapping private IP addresses to different port numbers attached to the single public IP address. When a computer within the private network sends a data packet to be routed to the Internet, the gateway replaces the source address with the single public IP address together with a random port number between 1024 and 65536 (Figure 2-2). When a data packet is returned with this destination Chapter Two Table 2-5: Example of a Simple PAT Table Private IP address Public IP address:Port 192.168.0.1 129.35.78.178:2001 192.168.0.2 129.35.78.178:2002 192.168.0.3 129.35.78.178:2003 192.168.0.4 129.35.78.178:2004 address and port number, the PAT table (Table 2-5) enables the gateway to route the data packet to the originating computer in the private network. Data Link Layer Technologies The Data Link layer is divided into two sub-layers - Logical Link Control (LLC) and Media Access Control (MAC). From the Data Link layer down, data packets are addressed using MAC addresses to identify the specific physical devices that are the source and destination of packets, rather than the IP addresses, URLs or domain names used by the higher OSI layers. Logical Link Control Logical Link Control (LLC) is the upper sub-layer of the Data Link layer (Figure 2-3), and is most commonly defined by the IEEE 802.2 standard. It provides an interface that enables the Ne