Why is google showing me a different external IP address than router

This question is in a way a follow up question from here: Can an ISP block port forwarding?

I was look at the isp router's info page and the IPv4 address is :

IPv4 Address
100.87.***.1**

However when when I google what is my ip address i get: 223.***.**.15

I am confused because if i now use 100.87.***.1** i am able to connect to my SoftEther server using the TCP protocol. But google is showing me a different IP address. So my question is two fold:

• Why is google showing me a different ip address? Clearly 100.87.**.*** is my real ip address.
• All my other services work on my local machine when i specify my external IP address but if i use mobile network and specify the port say 100.87.**.***:21, why am I being rejected.

100.64.0.0/10 is shared address space.

This means your ISP is doing CGNAT.

That IP address you see on your router is internal to your ISP and is not being propagated out onto the general internet.

Your ISP is mapping 100.87.***.1** onto 223.***.**.15.

It also means you cannot use port forwarding, as that has to be done at your ISP edge boundary initially. Well, unless you ask them very nicely.

You could ask your ISP for a proper, routable IP address, but that is likely going to cost you.