Cisco Simple NAT

WHAT

The simplest way to learn some cisco is trying to do a simple NAT in your home router so you can share your internet connection multiple computers. This is probably the most asked question by beginners.

STEP 1 - Gateway

Get into configuration mode

 Router>enable
 Router#configure terminal
 Enter configuration commands, one per line.  End with CNTL/Z.

Set the hostname so we don't get confused

 Router(config)#hostname gateway

And configure the fa0/0 interface as the inside nat network

 gateway(config)#interface FastEthernet 0/0
 gateway(config-if)#ip nat inside
 gateway(config-if)#ip address 192.168.0.1 255.255.255.0
 gateway(config-if)#no shutdown
 *Mar  1 00:18:38.071: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
 *Mar  1 00:18:39.071: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
 gateway(config-if)#exit

Same thing for 1/0, outside nat ( our Inet connection )

 gateway(config)#interface FastEthernet 1/0
 gateway(config-if)#ip nat outside
 gateway(config-if)#ip address 10.0.0.3 255.255.255.0
 gateway(config-if)#no shutdown
 *Mar  1 00:18:40.011: %LINK-3-UPDOWN: Interface FastEthernet1/0, changed state to up
 *Mar  1 00:18:40.013: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/0, changed state to up
 gateway(config-if)#exit

Now set up a standard accesslist with the name 'inside_nat_addresses' ( feel free to change it ) in which we tell that 192.168.*.* will match this ACL

 gateway(config-if)#ip access-list standard inside_nat_addresses
 gateway(config-std-nacl)#permit 192.168.0.0 0.0.0.255
 gateway(config-std-nacl)#exit

NOW, GOD says to do NAT ! This is read "I want to nat from _inside out_ all the hosts in 'inside_nat_addresses' to fa1/0 with port forwarding

 gateway(config)#ip nat inside source list inside_nat_addresses interface FastEthernet 1/0 overload
 gateway(config)#exit
 *Mar  1 00:18:56.083: %SYS-5-CONFIG_I: Configured from console by console

SAVE it and pray it works ! ( WORKED FOR ME !

 gateway#write mem
 Building configuration...
 [OK]

STEP 2 - Client

Now 1 of the clients .. I'll be using another router as a client ( cisco again )

 Router>enable
 Router#configure terminal
 Enter configuration commands, one per line.  End with CNTL/Z.

Set the hostname so we don't get confused

 Router(config)#hostname test

And configure the fa0/0 interface as one ip in our NAT network

 test(config)#interface FastEthernet 0/0
 test(config-if)#ip address 192.168.0.5 255.255.255.0
 test(config-if)#no shutdown
 *Mar  1 00:20:11.051: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
 *Mar  1 00:20:12.061: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
 test(config-if)#exit

Now add the default gateway

 test(config)#ip route 0.0.0.0 0.0.0.0 192.168.0.1
 test(config)#exit

Et voila !

 test#write mem
 Building configuration...
 [OK]

DEBUGGING

Now you should be able to NAT all your 192.168.*.* to your fa1/0

Check the current settings using

 show ip interface brief 

and the nat connections

 show ip nat translations

Some debugging

 debug ip packet detail

and

 debug ip nat detail

Be careful with the debug levels, 'detail' is only recommended in lab situations !

When trying to use pings, to stop them use ctrl+shift+6