Static NAT on Cisco IOS

Let’s take a look at how to configure static NAT on a Cisco router. Here’s the topology I will use:

Above you see 3 routers called Host, NAT and Web1. Imagine our host is on our LAN and the webserver is somewhere on the Internet. Our NAT router in the middle is our connection to the Internet.

There’s a cool trick on our routers that we can use. It’s possible to disable “routing” on a router which turns it into a normal host that requires a default gateway. This is very convenient because it will save you the hassle of connecting real computers/laptops to GNS3.

Host(config)#no ip routing

Web1(config)#no ip routing

Use no ip routing to disable the routing capabilities. The routing table is now gone, let me show you:

Host#show ip route 
Default gateway is not set

Host               Gateway           Last Use    Total Uses  Interface
ICMP redirect cache is empty

Web1#show ip route 
Default gateway is not set

Host               Gateway           Last Use    Total Uses  Interface
ICMP redirect cache is empty

As you can see the routing table is gone. We’ll have to configure a default gateway on router Host and Web1 or they won’t be able to reach each other:

Host(config)#ip default-gateway 192.168.12.2

Web1(config)#ip default-gateway 192.168.23.2

Both routers can use router NAT as their default gateway. Let’s see if they can reach each other:

Host#ping 192.168.23.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.23.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/8/12 ms

Reachability is no issue as you can see. Now let me show you a neat trick:

Web1#debug ip packet 
IP packet debugging is on

I can use debug ip packet to see the IP packets that I receive. DON’T do this on a production network or you’ll be overburdened with traffic! Now let’s send that ping again…

Web1#
IP: s=192.168.12.1 (FastEthernet0/0), d=192.168.23.3, len 100, rcvd 1

Above you see that our router has received an IP packet with source IP address 192.168.12.1 and destination IP address 192.168.23.3.

IP: tableid=0, s=192.168.23.3 (local), d=192.168.12.1 (FastEthernet0/0), routed via RIB

And it will reply with an IP packet that has source address 192.168.23.3 and destination address 192.168.12.1.

Now let’s configure NAT so you can see the difference:

NAT(config)#interface fastEthernet 1/0
NAT(config-if)#ip nat inside

NAT(config)#interface fastEthernet 0/0
NAT(config-if)#ip nat outside

First we’ll have to configure the inside and outside interfaces. Our host is the “LAN” side so it’s the inside. Our webserver is “on the Internet” so it’s the outside of our network. Now we can configure our static NAT rule: