Black Box LR1102A-T1/E1 Configuring NAT, 12.1Network Address Translation, Dynamic NAT, Static NAT

12

CONFIGURING NAT

12.1Network Address Translation

Network Address Translation (RFC 1631) is commonly known as NAT. This application discusses NAT and provides a technical explanation and configuration examples.

Features:

„Dynamic Address/Port Translation

„Static Address/Port Translation

„Forward and Reverse NAT

„Non-Translated Address Pass Through

In the most common NAT application, the device (Black Box system) that connects the user LAN to the Internet will have two IP addresses:

„A private IP address on the LAN side for the RFC 1918 address range

„A public address, routable over the Internet, on the WAN side

Consider a PC on the LAN sending a packet destined for some.server.com. The source IP address and port are in the packet together with the destination IP address and port. When the packet arrives at the Black Box system it will be de-encapsulated, modified, and re-encapsulated. The re-encapsulated packet sent by the Black Box system destined for the Internet contains the Black Box system’s public IP address, a source port allocated from its list of available ports, and the same destination IP address and port number generated by the PC. The Black Box system also adds an entry into a table it keeps, which maps the internal address and source port number that the PC generated against the port number it allocated to this session. Therefore, when some.server.com sends a reply packet to the PC, the Black Box system can quickly determine how it needs to re-write the packet before transmitting it back on to the LAN.

12.1.1 Dynamic NAT

Dynamic NAT is used when packets destined for the Internet are transported from a LAN using the public source IP address assigned to the local router. Dynamic NAT performs this task well, but it does not permit providing services to the Internet from inside a LAN. In these instances, static NAT is used.

12.1.2 Static NAT

Static NAT also requires a public address from the upstream service provider. Individual PCs within a LAN are assigned RFC 1918 reserved IP addresses to enable access to other PCs within the LAN. The Black Box system is configured with static mapping, which maps the internal RFC 1918 IP addresses for each PC to the appropriate public IP address. Then when traffic is sent to the public address listed in the static mapping, the Black Box system forwards the packets to the correct PC within the LAN, according to the mapping relationship established.