Although I understand how NAT is configured on Cisco, a broader question came up about the why behind it.
When configuring e.g. NAT overload, you specify a few items:
An access list ...
ip access-list extended LAN_ACL permit ip 192.168.0.0 0.0.0.255 any
...and a NAT configuration itself:
ip nat inside source list LAN_ACL interface Fa0 overload
Besides that, you obviously need connections to the network specified:
interface Vlan1 ip address 192.168.0.1 255.255.255.0 interface Fa0 ip address 203.0.113.23 255.255.255.0
What I do not understand fully is as to why you need to further specify
ip nat inside and
ip nat outside on the interfaces.
The interface name is mentioned in the NAT statement and the network of the ACL must be unique on the router (VRF technology excepted). Can the system not determine which is in- and out-side by what is given?
Any insight is appreciated.