My CISCO Journal

In this part we are going to configure Dynamic NAT on our gateway router ( R2). We will following the previously mentioned three steps to achieve this task like: 

So here we go, and you will see it’s not that difficult to implement it practically:

Dynamic NAT Configurations:

Suppose we want to configure Dynamic NAT on R2. For this purpose, we need a pool of global IP addresses that would be dynamically mapped with our local hosts. So we need to buy these addresses from our ISP J just assume, we bought the following IP pool form our ISP:

171.16.10.52 - 171.16.10.56

We will follow above mentioned three steps to implement dynamic NAT on our current network topology:

Step 1:

Labeling the interfaces:

interface FastEthernet0/0

 ip address 192.168.1.2 255.255.255.0

 ip nat inside

interface Serial1/0

 ip address 171.16.10.1 255.255.255.0

 ip nat outside

Step 2:

An ACL needed to be created for local hosts IPs that we want to translate:

ip access-list standard NAT_IPs

 permit 192.168.3.0 0.0.0.255

 permit 192.168.2.0 0.0.0.255

 permit 192.168.4.0 0.0.0.255

Step 3:

Once ACL is created, we need to configure our IP pool and dynamic NAT from global configuration like:

ip nat pool Global_IP_Pool 171.16.10.52 171.16.10.56 netmask 255.255.255.0

ip nat inside source list NAT_IPs pool Global_IP_Pool

Our pool name is: Global_IP_Pool

ACL name is: NAT_IPs

Some of the other things done on R2 are: static route to ISP and RIP:

!

router rip

 passive-interface Serial1/0 ( this commands is configured to stop RIP advertisements to our emulated ISP)

 network 171.16.0.0

 network 192.168.1.0

!

ip route 171.16.0.0 255.255.0.0 171.16.10.2

!

!

And we are done with our Dynamic NAT! that was quite easy and simple. Now some other fun commands. In order to check the IP NAT translations going on in the network, we just issue show ip nat translations command on R2 and see the result:

From R1 we ping our ISP:

R1#ping 171.16.10.2 source 192.168.3.1

We get successful ping results to 171.16.10.2. The source IP was changed on R2 during the process of pinging as we have configured NAT on R2.

Now let’s see what’s happening on R2:

I hope after going through all of the above commands, you will have confidence in yourself and will see how interesting it is to implement.  Just remember one thing: if dynamic NAT is used, we can’t access our local hosts from outside the network, as the router will not be sure to where it has to route the packets and for this purpose Static NAT is recommended, which we will discuss shortly.

Okay, as an example, please perform some practice of dynamic NAT with following requirements:

Static NAT: static NAT is a one to one mapping of internal addresses with the global IP addresses. In network design, static NAT is best if we are going to host some server inside our network and want it to be accessible from global internet.

Some of the terminology of NAT worth remembering: