Network Address Translation (NAT) was introduced to overcome these addressing problems that occurred with the rapid expansion of the Internet. Basically NAT allows a single network device (e.g a router, firewall etc) to act as an agent between a private local area network and a public network such as the Internet. The purpose of this NAT device is to translate the source IP addresses of the internal network hosts into public routable IP addresses in order to communicate with the Internet.

::Advantages::

• NAT helps to mitigate the depletion of the global public IP address space
• Networks can now use the RFC 1918 private address space internally and still have a way to access the Internet using NAT.
• NAT increases security by hiding the internal network topology and addressing scheme.

Cisco IOS routers support different types of NAT as will be explained below. NAT has many forms and can work in several ways, but in this post I will explain the four most important types of NAT:

1. Overloading or Port Address Translation (PAT)

This is the most frequently used form of NAT in IP networks. It uses the concept of “many-to-one” translation where multiple connections from different internal hosts are “multiplexed” into a single registered (public) IP address using different source port numbers. This type of NAT allows a maximum of 65,536 internal connections to be translated into a single public IP. This type of NAT is very useful in situations where our ISP has assigned us only a single public IP address, as shown below.



In our scenario above, our internal network range is 192.168.32.0/24 and our assigned public IP address is 213.18.123.100. All internal hosts will be translated to the public address using different port numbers.

Configuration:

Router(config)# interface ethernet 0
Router(config-if )# ip address 192.168.32.1 255.255.255.0
Router(config-if )# ip nat inside

Router(config)# interface serial 0
Router(config-if )# ip address 213.18.123.100 255.255.255.0
Router(config-if )# ip nat outside

Router(config)# ip nat pool overloadpool 213.18.123.100 213.18.123.100 prefix-length 24
Router(config)# ip nat inside source list 1 pool overloadpool overload
Router(config)# access-list 1 permit 192.168.32.0 0.0.0.255

2. Dynamic NAT

Dynamic NAT translates internal private IP addresses to public addresses from a range (pool) of public addresses assigned to our network from an ISP.



In our example scenario above, assume that we own the range of public IP addresses 213.18.123.0/24. Any internal host accessing the internet, will be translated by the NAT router to the first available public IP in the public pool range. In our example above, internal host 192.168.32.10 is translated to 213.18.123.116 (one-to-one mapping). Similarly, 192.168.32.12 is translated to 213.18.123.112 etc.

Configuration:

Router(config)# interface ethernet 0
Router(config-if )# ip address 192.168.32.1 255.255.255.0
Router(config-if )# ip nat inside

Router(config)# interface serial 0
Router(config-if )# ip address 100.100.100.1 255.255.255.252
Router(config-if )# ip nat outside

Router(config)# ip nat pool dynamicpool 213.18.123.0 213.18.123.255 prefix-length 24
Router(config)# ip nat inside source list 1 pool dynamicpool
Router(config)# access-list 1 permit 192.168.32.0 0.0.0.255

3. Static NAT

This form of NAT creates a permanent one-to-one static mapping of a public IP address with a private IP address. It is particularly useful in cases where an internal host needs to be accessible from the outside public internet.



In our example diagram above, the internal host with private IP address 192.168.32.10 will always be translated to 213.18.123.110. Hosts from the outside public internet will be able to directly access the statically nated internal hosts by accessing their mapped public IP address. This scenario is useful to provide access to public company servers such as Web Server, Email Server etc.

Configuration:

Router(config)# interface ethernet 0
Router(config-if )# ip address 192.168.32.1 255.255.255.0
Router(config-if )# ip nat inside

Router(config)# interface serial 0
Router(config-if )# ip address 100.100.100.1 255.255.255.252
Router(config-if )# ip nat outside

Router(config)# ip nat inside source static 192.168.32.10 213.18.123.110
Router(config)# ip nat inside source static 192.168.32.12 213.18.123.111
Router(config)# ip nat inside source static 192.168.32.15 213.18.123.112

4. Port Redirection

This is useful in situations where we have a single public IP address and we need to use it for accessing two or more internal servers from outside. Assume that we have a Web and Email servers that we need to provide access from outside using only a single public IP address. Assume that our public address is 100.100.100.1. Inbound traffic coming towards address 100.100.100.1 port 80 will be redirected to our internal Web Server 192.168.32.10, and inbound traffic coming towards address 100.100.100.1 port 25 will be redirected to our internal Email Server 192.168.32.20.

Configuration:

Router(config)# interface ethernet 0
Router(config-if )# ip address 192.168.32.1 255.255.255.0
Router(config-if )# ip nat inside

Router(config)# interface serial 0
Router(config-if )# ip address 100.100.100.1 255.255.255.252
Router(config-if )# ip nat outside

Router(config)# ip nat inside source static tcp 192.168.32.10 80 100.100.100.1 80
Router(config)# ip nat inside source static tcp 192.168.32.20 25 100.100.100.1 25


Troubleshooting commands:

Router#show ip nat translation

To show the current translations preformed by NAT

Router#show ip nat static

To show the static translations of NAT

Router#debug ip nat

To watch the instantaneous interactions of NAT


Note: To disable NAT, you need to do the following steps:

1. Disable NAT on the local and global interfaces

Router(config-if)#no ip nat inside

on the local, and

Router(config-if)#no ip nat outside

on the global interface.

2. Clear the contents of the translation table,

Router#clear ip nat translations

3. Remove the NAT assignment command by preceding it with a ‘no’

4. Remove the access-list, if any.