The difference between the two is that the first command is not concerned with what direction the traffic is coming from (inside, outside, sideways). It will figure out what interface it came in on and going out off and nat accordingly. The second nat command is concerned with what interface you have configured as inside and outside and will translate based off of those configurations. For example:
With "ip nat source list 100 interface f0/0 overload":
int fa0/0
ip nat enable
int fa0/1
ip nat enable
With "ip nat inside source list 100 interface fa0/0 overload":
int fa0/0
ip nat outside
int fa0/1
ip nat inside
I still use the older configuration of "ip nat inside" most times. There is one other differnence, and that's the way that you see the translations. The "ip nat source" command will create an NVI interface. To see translations, you would do:
show ip nat translations (for ip nat inside source command)
or
show ip nat nvi translations (for ip nat source command)
So there is a difference. The "ip nat source" is considered easier because you don't have to take into account which direction interfaces may be receiving traffic in. You can have two outside interfaces, one inside, or five inside interfaces and two outside. With the "ip nat enable" command on those interfaces, the router does the work...
HTH,
John
HTH,
John
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