Why Is nat overload necessary?

[Iloveyou]

What happens if we put ip nat outside and ip nat inside BUT we DONT put 

ip nat inside source list 1 interface FastEthernet4 overload
!
access-list 1 permit 192.168.1.0   0.0.0.255

From my logical understanding, if you configure NAT on the interfaces, the router should know what to do even if you dont overload the interface. So why is NAT overloading and ACL necessary?

[MHM Cisco World]

first this way of study right way 
second 
even if you not add overload the router automatically will add it. 
for ip nat inside vs ip nat outside 
first one NATing the source of packet (go from inside to outside)
second one NATing the destination of packet (go from inside to outside)

[Iloveyou]

I think we have to put the access list and the dynamic overload command before it works?

No put access list, no put dynamic overload command no work?

I think dynamic overload command not automatic put.

[MHM Cisco World]

I think we have to put the access list and the dynamic overload command before it works? Yes, 
even if you not add overload the router automatically will add it. <<- what I meaning that if you config ip nat inside list...... without add overload keyword the router automatically Will add it.

""No put access list, no put dynamic overload command no work?
I think dynamic overload command not automatic put.""
please elaborate more about these two points 

[Iloveyou]

I just put ip nat inside and ip nat outside on the interfaces.

The dynamic overload command does not automatically appear.

[MHM Cisco World]

you can see I config ip nat inside without overload 
when I do show run 
there is overload 
so Router add it automatically

[MHM Cisco World]

You post have many Q' I hope overload keyword is now clear to you.

Other Q

If you add to interface ip nat inside/outside 

But 

I DONT config ip nat inside source static/list what happened?

Simply the traffic will flow without any NATing.

[Iloveyou]

But why it no work.

On LAN interface i put ip nat inside

on wan interface i put ip nat outside

I no put acl and i no put "ip nat inside source static/list".

Why NAT will not automatic happen?

[MHM Cisco World]

Ok' these examples show you why you must add ip nat inside static/list 

1- You have one ip nat inside interface 

And three ip nat outside 

If traffic is receive and need to NATing for which outside it automatically NATing ??

2-

Lan's -sw- router1-router2

Router2 have one inside interface and one outside interface 

Router2 through it one inside interface recive traffic from all lan's (different subnet) and need automatically NATing traffic' which one will NATing and which not.

That why cisco router and sure other vendor config same 

First config NAT boundary with ip nat inside and ip nat outside 

Then config guide for nating with 

Ip nat inside source static/list