Dynamic NAT

Everything has its own types and each and every type of it is of different kind. Similarly, NAT`s another type is dynamic NAT. Dynamic NAT is similar to static NAT in that the NAT router creates a one-to-one mapping between an inside local and inside global address and changes the IP addresses in packets as they exit and enter the inside network.

However, the mapping of an inside local address to an inside global address happens dynamically. Dynamic NAT accomplishes this by setting up a pool of possible inside global addresses and defining criteria for the set of inside local IP addresses whose traffic should be translated with NAT.

With dynamic NAT, you can configure the NAT router with more IP addresses in the inside local address list than in the inside global address pool. When the number of registered public IP addresses is defined in the inside global address pool, the router allocates addresses from the pool until all are allocated. If a new packet arrives, and it needs a NAT entry, but all the pooled IP addresses are already allocated, the router discards the packet. The user must try again until a NAT entry times out, at which point the NAT function works for the next host that sends a packet. This can be overcome through the use of Port Address Translation (PAT) which allows many hosts to be mapped to a single address at the same time.

When PAT is configured, the router makes use of the source port numbers to distinguish the sessions from different hosts. PAT creates an extended translation entry in the NAT table by including the protocol as well as the port information.

PAT is configured by adding the overload option to the dynamic NAT configuration command, which binds the hosts and the pool.
Static and dynamic NAT can be configured simultaneously when it is required.