Chapter 22: Network Address Translation; Dynamic Nat; Static Nat - Avaya 1000 Series Configuration Manual

Chapter 22: Network 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 (Secure Router) 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, able to be routed 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
Secure Router it will be de-encapsulated, modified, and re-encapsulated. The re-encapsulated packet
sent by the Secure Router destined for the Internet contains the Secure Router'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 Secure Router 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 Secure Router can quickly
determine how it needs to rewrite the packet before transmitting it back on to the LAN.

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.

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
Avaya Secure Router 1000 Series Configuration Guide December 2010 107