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:

static nat inside outside

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:

We're Sorry, Full Content Access is for Members Only...

If you like to keep on reading, Become a Member Now! Here is why:

  • Learn any CCNA, CCNP and CCIE R&S Topic. Explained As Simple As Possible.
  • Try for Just $1. The Best Dollar You've Ever Spent on Your Cisco Career!
  • Full Access to our 660 Lessons. More Lessons Added Every Week!
  • Content created by Rene Molenaar (CCIE #41726)

505 Sign Ups in the last 30 days

satisfaction-guaranteed
100% Satisfaction Guaranteed!
You may cancel your monthly membership at any time.
No Questions Asked!

Tags: ,


Forum Replies

  1. Hello Rene.

    Fantastic lesson. I have some doubts about NAT like the one described below using your example:

    When a ping is done from the NAT server to the Webserver there is a failure.

    Logs from NAT are reporting this:

    NAT#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:
    
    *Mar  1 00:12:43.191: IP: tableid=0, s=192.168.23.2 (local), d=192.168.23.3 (FastEthernet0/1), routed via FIB
    *Mar  1 00:12:43.191: IP: s=192.168.23.2 (local), d=192.168.23.3 (FastEthernet0/1), len 100, sending
    *Mar  1 00:1
    ... Continue reading in our forum

  2. Hello AZM

    The ARP request would come from the 3560 router saying “I need the MAC address of the dev

    ... Continue reading in our forum

  3. the following config is in the C891F-K9 in a production network my Question is why they config the ip nat static with route-map option

      router#sh running-config interface g8
    Building configuration...
    
    Current configuration : 357 bytes
    !
    interface GigabitEthernet8
     description WAN
     bandwidth 512
     bandwidth receive 3000
     ip address x.x.x.x 255.255.255.248
     ip access-group BLOCK_RECURSIVE in
     ip nat outside
     ip ips myips in
     ip virtual-reassembly in
     zone-member security INTERNET-ZONE
     duplex auto
     speed auto
     crypto map primarymap
     service-policy output MP_STD_4
    ... Continue reading in our forum

37 more replies! Ask a question or join the discussion by visiting our Community Forum