Configure NAT – GNS3 Lab

In this article we will demonstrate how to configure NAT using GNS3

Note: If you are not sure about NAT, please read my Network Address Translation NAT Tutorial

To configure static NAT we need to complete these tasks:
* Define the router’s interfaces as inside or outside:
R0uter(config-if)#ip nat inside (or ip nat outside)

* Define static mapping between the inside address and the outside address:
R0uter(config)#ip nat inside source static

+ Static NAT:

To make everything clear, we will configure static NAT in GNS3. Open your GNS3 and build a topology like this:

(IOS used: c2600-bin-mz.123-6f.bin but you can use other versions)

We should use 3 routers in this topology but I want to save some RAM and demonstrate how to ping from the loopback interface so I only use two :) Therefore we should configure the loopback interface of R0 as the source IP address and the fa0/0 interface of R0 as the “outgoing static NAT” address.

R0#configure terminal
R0(config)#int loopback0
R0(config-if)#ip address 10.0.0.1 255.0.0.0
R0(config-if)#ip nat inside

R0(config-if)#int f0/0
R0(config-if)#ip address 200.0.0.1 255.255.255.0
R0(config-if)#no shutdown
R0(config-if)#ip nat outside
R0(config-if)#exit

Finally, we have to tell the router to translate my private IP 10.0.0.1 to public IP 200.0.0.2 so that I can go to the Internet!

R0(config)#ip nat inside source static 10.0.0.1 200.0.0.2

In R1 we just assign the IP address and no shut its interface.

R1#config terminal
R1(config)#int f0/0
R1(config-if)#ip address 200.0.0.10 255.255.255.0
R1(config-if)#no shutdown

Check if all things are right or not:

R0#show ip nat translations

In this article we don’t use a host attached to R0 so if we want to test our NAT configuration we have to ping from R0′s loopback interface by using the ping extended command:

We can use the extended ping command by typing only “ping” at the privileged mode, specify the “target IP address” and type “y” at the “Extended commands” and specify the “source address or interface” at shown below:

To approve NAT works well we can disable static NAT with the following command

R0(config)#no ip nat inside source static 10.0.0.1 200.0.0.2

Now if we use the extended ping command (without NAT configured):

-> We can’t ping from the loopback interface.

Download static NAT configuration: http://www.9tut.com/download/NAT_static_CCNA_self_study.zip

+ Dynamic NAT:

To configure dynamic NAT we need to complete these tasks:

* Define a pool of addresses (public IP) to be used for dynamic NAT allocation

Router(config)#ip nat pool pool_name start_ip end_ip { netmask netmask | prefix-length prefix-length }

* Configure a standard access control list to define what internal traffic will be translated

Router(config)#access-list access-list-number permit source [source-wildcard]

Link the access list to the NAT pool

Router(config)#ip nat inside source list access-list-number pool pool_name

Router(config-if)# ip nat inside (on fa0/0, for example)
Router(config-if)#ip nat outside (on fa0/1, for example)

* Dynamic NAT configuration example:

RouterA(config)# access-list 1 permit 192.168.0.0 0.0.0.255
RouterA(config)# ip nat pool PoolforNAT 200.23.123.6 200.23.123.10 netmask 255.255.255.0
RouterA(config)# ip nat inside source list 1 pool PoolforNAT

Note: In the above command, the word “inside” means “I want to NAT from inside to outside”; “list 1″ means “the source IP addresses to NAT are included in Access-list 1″; “pool PoolforNAT” means “NAT to the IP addresses specified in PoolforNAT”.

RouterA(config)# int loopback0
RouterA(config-if)# ip nat inside

RouterA(config-if)# int fa0/0
RouterA(config-if)# ip nat outside

Configure PAT (NAT Overload)

* Configure a standard access list to define what internal traffic will be translated
* Link the access list to the interface to be used for PAT
* Define interfaces as either inside or outside

PAT router commands
RouterA(config)# access-list 1 permit 192.168.0.0 0.0.0.255
RouterA(config)# ip nat inside source list 1 interface fa0/0 overload

(Notice the “interface fa0/0″ means “NAT out of this interface” and the keyword overload for PAT in the above command)

RouterA(config)# interface fa0/0
RouterA(config-if)# ip nat outside

RouterA(config-if)# interface loopback0
RouterA(config-if)# ip nat inside