An IP address pool is a group of IP addresses. You create an IP address pool by assigning a range of IP addresses and a name to it. You allocate or assign addresses in the pool to users.
The ability of Network Address Translation (NAT) to consistently represent a local IP address as a single global IP address is termed paired address pooling. A local address is any address that appears on the inside of a network, and a global address is any address that appears on the outside of the network. You can configure paired address pooling only for Port Address Translation (PAT) because dynamic and static NAT configurations are paired configurations by default. PAT, also called overloading, is a form of dynamic NAT that maps multiple, unregistered IP addresses to a single, registered IP address (many-to-one) by using different ports. Paired address pooling is supported in both classic (default) and carrier-grade NAT (CGN) mode.
In a paired-address-pooling configuration, a local address is consistently represented as a single global address. For example, if User A is paired with the global address G1, that pairing will last as long as there are active sessions for User A. If there are no active sessions, the pairing is removed. When User A has active sessions again, the user may be paired with a different global address.
If a local address initiates new sessions, and resources (ports) are insufficient for its global address, packets are dropped. When the number of users on a global address reaches the configured limit, the next global address is used for paired address pooling. When a user who is associated with a global address through paired address pooling is unable to get a port number, then the packet is dropped, the NAT drop code is incremented, and Internet Control Message Protocol (ICMP) messages are not sent.
Paired-address-pooling uses the fill-it-up method for address selection. The fill-it-up method fits (adds) the maximum possible users into a single global address before going to the next global address.