The packet gateway supports both network address translation (NAT) and network address port translation (NAPT). Twice NAT enables you to configure both source addresses and destination addresses that are translated as packets traverse the router. You can apply twice NAT for VoIP packets (signaling and media) as they traverse gates to achieve security between realms or service providers. To apply twice NAT, the pgcpd process instructs the PIC to allocate a specified number of NAT addresses and ports from a PGCP NAT pool on a per-gate basis. The pgcpd process specifies which NAT pool to use.
Figure 18 shows two gates in a packet gateway.
Figure 18: Translation of Gate Addressing
After flows are created for Gate 1, the gate connects the remote source to the local destination. The local source and local destination addresses reside on the router and must be uniquely specified. For Gate 1, twice NAT enables the router to translate the IP address of the remote source to the local source, and the local destination to the remote destination.
To create the bidirectional flow, the same IP address is used for the local source in Gate 2 and the local destination in Gate 1. Likewise, the same IP address is used for the remote source in Gate 1 and the remote destination in Gate 2.
Figure 19 shows an example of how addresses are translated.
Figure 19: Example: Translation of Gate Addressing
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