Dynamic NAT allows routers to dynamically map private IP addresses to public IP addresses from a pool. The router chooses an available public IP address for outgoing connections and tracks the mapping in its NAT table. When responses come back, it reverses the mapping to deliver packets to the original private IP address. Configuring dynamic NAT involves defining the internal and external interfaces, an ACL for matched addresses, a global address pool, and enabling NAT with the ACL and pool.

1. Dynamic NAT

2.  With dynamic NAT, you specify two sets of addresses on your
Cisco router:
1. inside addresses that will be translated
2. a pool of global addresses
 Unlike with static NAT, where you had to manually define a static
mapping between a private and a public address, with dynamic NAT
the mapping of a local address to a global address happens
dynamically.
 This means that the router dynamically picks an address from the
global address pool that is not currently assigned. It can be any
address from the pool of global addresses.
 The dynamic entry stays in the NAT translations table as long as the
traffic is exchanged. The entry times out after a period of inactivity
and the global IP address can be used for new translations.

3.  To configure dynamic NAT, the following steps are required:
1. configure the router's inside interface using the ip nat
inside command
2. configure the router's outside interface using the ip nat
outside command
3. configure an ACL that has a list of the inside source addresses
that will be translated
4. configure the pool of global IP addresses using the ip nat pool
NAME FIRST_IP_ADDRESS LAST_IP_ADDRESS netmask
SUBNET_MASK command
5. enable dynamic NAT with the ip nat inside source list
ACL_NUMBER pool NAME global configuration command

4. Here is an example.

5.  Computer A requests a web resource from S1.
 Computer A uses its private IP address when sending the
request to router R1.
 Router R1 receives the request, changes the private IP
address to one of the global addresses in the pool and sends
the request to S1.
 S1 responds to R1. R1 receives the response, looks up in its
NAT table and changes the destination IP address to the
private IP address of Computer A.

6.  In the example in a previous slide we need to configure
dynamic NAT. To do that, the following commands are
required on R1:
 To configure the router's inside interface:
 To configure the router's outside interface:

7.  To configure an ACL that has a list of the inside source
addresses that will be translated:
 NOTE – the access list configured above matches all hosts
from the 192.168.0.0/24 subnet.
 To configure the pool of global IP addresses:
 NOTE – the pool configured above consists of 5 addresses:
4.4.4.1, 4.4.4.2, 4.4.4.3, 4.4.4.4, and 4.4.4.5.

8.  To enable dynamic NAT:
 The command above tells the router to translate all
addresses specified in the access list 1 to the pool of global
addresses called MY_POOL.

9.  You can list all NAT translations using the show ip nat
translations command:
 In the example above, you can see that the private IP
address of Computer A (192.168.0.2) has been translated to
the first available global address (4.4.4.1).
 NOTE – you can remove all NAT translations from the table
by using the clear ip nat translation * command.

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