NAT OVERLOAD ON CISCO ROUTER

NAT Overloading also called Port Address Translation (PAT) is form of dynamic NAT where we have is just a single inside global IP address providing Internet access to all inside hosts. As a general case, NAT Overload is used in scenarios where the number of inside local addresses is greater than the number of inside global addresses.

A simple scenario of NAT Overload configuration will help audience have better understanding of Network address Translation concept and traffic flow across network elements.

nat-overload-on-cisco-router

In the setup, R1 and R2 routers in Post to Post Links II error: Unrecognized type: ”slug” have been configured as end systems (host machines) which are connected through a Layer 2 Post to Post Links II error: Unrecognized type: ”slug” (SW) to customer Gateway Router (R3). The Gateway Router is further connected to Internet Service provider (ISP).The customer has been assigned Public IP address by ISP as below –

  • Set of Public IP 200.200.200.0/30 for WAN Connectivity to ISP
  • Additional Public IP of 100.100.100.1 for customer access to Internet.

In order for multiple LAN Users (192.168.123.0/24) to access Internet via Single Public IP i.e. 100.100.100.1, NAT feature of “NAT Overload” will be used here. NAT Overload, also known as PAT (Port Address Translation) is essentially NAT with the added feature of TCP/UDP ports translation.

The configuration for each device is shown below –

R1 Configuration

R1(config)#int fa0/0
R1(config-if)#ip addr 192.168.123.1 255.255.255.0
R1(config-if)#no sh
R1(config-if)#exit
R1(config)#no ip routing
R1(config)#ip default-gateway 192.168.123.3

R2 Configuration
R2(config)#int fa0/0
R2(config-if)#ip addr 192.168.123.2 255.255.255.0
R2(config-if)#no sh
R2(config-if)#exit
R2(config)#no ip routing
R1(config)#ip default-gateway 192.168.123.3

R3 Basic configuration –
R3(config)#int fa0/0
R3(config-if)#ip addr 192.168.123.3 255.255.255.0
R3(config-if)#no sh
R3(config)#int fa0/1
R3(config-if)#ip addr 200.200.200.1 255.255.255.252
R3(config-if)#no sh
R3(config-if)#exit
R3(config)#ip route 0.0.0.0 0.0.0.0 200.200.200.2

Now , we will configure R3 to perform NAT Overload as below –
R3(config)#ip nat pool NATPOOL 100.100.100.1 100.100.100.1 netmask 255.255.255.0
R3(config)#ip access-list standard LANPOOL permit 192.168.123.0 0.0.0.255
R3(config)#ip nat inside source list LANPOOL pool NATPOOL overloadR3(config)#int fa0/0
R3(config-if)#ip nat inside
R3(config)#int fa0/1
R3(config-if)#ip nat outside
R3(config-if)#exit

nat-overload-on-cisco-router

Once the NAT Overload configuration is complete, we will verify the same –

On R1 (ping any Global IP – In this case lets say 4.4.4.4) –

R1# ping 4.4.4.4
R1(config)#do ping 4.4.4.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 4.4.4.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 88/105/124 ms
Now , lets view the R3 NAT Transalation table –
R3#sh ip nat translation
Pro     Inside global      Inside local       Outside local      Outside global
icmp 100.100.100.1:5   192.168.123.1:5    4.4.4.4:5          4.4.4.4:5
The same ping is performed on R2 as below –
R3#sh ip nat translation
Pro    Inside global      Inside local       Outside local      Outside global
icmp 100.100.100.1:5   192.168.123.1:5    4.4.4.4:5          4.4.4.4:5
icmp 100.100.100.1:0   192.168.123.2:5    4.4.4.4:5          4.4.4.4:0
Because these entries are all dynamically created, they are temporary and will be removed from the translation table after some time.Note – Router WAN Interface (200.200.200.1) can also be used as Inside Global Address for NAT Overload instead of dedicated IP address (in this case 100.100.100.1).
Some more useful NAT commands are –
clear ip nat translation *
show ip nat statistics

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