PAT (also called NAT overload) conserves addresses in the inside global address pool by allowing the router to use one inside global address for many inside local addresses. In other words, a single public IPv4 address can be used for hundreds, even thousands of internal private IPv4 addresses. When this type of translation is configured, the router maintains enough information from higher-level protocols, TCP or UDP port numbers, for example, to translate the inside global address back into the correct inside local address. When multiple inside local addresses map to one inside global address, the TCP or UDP port numbers of each inside host distinguish between the local addresses.
Note: The total number of internal addresses that can be translated to one external address could theoretically be as high as 65,536 per IP address. However, the number of internal addresses that can be assigned a single IP address is around 4,000.
There are two ways to configure PAT, depending on how the ISP allocates public IPv4 addresses. In the first instance, the ISP allocates more than one public IPv4 address to the organization, and in the other, it allocates a single public IPv4 address that is required for the organization to connect to the ISP.
Configuring PAT for a Pool of Public IP Addresses
If a site has been issued more than one public IPv4 address, these addresses can be part of a pool that is used by PAT. This is similar to dynamic NAT, except that there are not enough public addresses for a one-to-one mapping of inside to outside addresses. The small pool of addresses is shared among a larger number of devices.
Figure 1 shows the steps to configure PAT to use a pool of addresses. The primary difference between this configuration and the configuration for dynamic, one-to-one NAT is that the overload keyword is used. The overload keyword enables PAT.
The example configuration shown in Figure 2 establishes overload translation for the NAT pool named NAT-POOL2. NAT-POOL2 contains addresses 126.96.36.199 to 188.8.131.52. Hosts in the 192.168.0.0/16 network are subject to translation. The S0/0/0 interface is identified as an inside interface and the S0/1/0 interface is identified as an outside interface.
Use the Syntax Checker in Figure 3 to configure PAT using an address pool on R2.