problem in NAT and echo reply

[engahmedsaied]

I can't ping loopback interface 192.168.137.1 from R1 I can do that from R2 and virtual machine too.
I already define routes ,I tried to make NAT for 10.123.0.4 to be 192.168.137.100 any ip in 192.168.137.0 network and this works fine but why ?!
why I need NAT and virtual machine can go to internet without this NAT
and loopback transfer all requests to my physical Network card interface and it go to my gateway (home router) and it is doing NAT

so I can't understand this 100%

why R1 can't not ping loopback interface 192.168.137.1 without NAT ?!!!

[nghia tran]

when you ping 192.168.137.1 from r1, the source ip is 10.123.0.4. 

I think you cant ping 192.168.137.1 from r1 because there is not a route back to 10.123.0.4 in the configuration of 192.168.137.1... 

192.168.137.1 might have only a default route to *other direction (outbound to the internet).