Bidirectional (2-Way) NAT

Traditional Outbound NAT is designed to handle outbound connections, in which clients of the inside local network initiate requests to outside global Internet hosts. Bidirectional NAT, 2-Way NAT, or Inbound NAT is an enhancement upon NAT to handle connections initiated from the outside network.

The network setup on the figure above can be achieved using a single router. PC1 accesses PC2 using 172.16.1.3 outside local; while PC2 accesses PC1 using 192.168.1.3 inside global.

Bidirectional NAT configuration on RT1:

!
interface FastEthernet0/0
 ip address 172.16.1.1 255.255.255.0
 ip nat inside
!
interface FastEthernet1/0
 ip address 192.168.1.1 255.255.255.0
 ip nat outside
!
ip nat inside source static 172.16.1.2 192.168.1.3
ip nat outside source static 192.168.1.2 172.16.1.3 add-route
!

The add-route keyword adds a static /32 host route for the outside local address. This route is used for routing and translating packets that travel from the inside to the outside of the network.

RT1#sh ip route

Gateway of last resort is not set

     172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C       172.16.1.0/24 is directly connected, FastEthernet0/0
S       172.16.1.3/32 [1/0] via 192.168.1.2
C    192.168.1.0/24 is directly connected, FastEthernet1/0
RT1#