Why different IP behind ISP modem

I have a simple setup:

internet <-- ISP modem (bridge mode) <-- openWrt router (router mode)

I left the wifi open on the ISP modem, so I can connect to the internet via my openWrt router or ISP router.

When I go to this website to check my IPv4 and IPv6 address, I was surprised to see:

  1. When connecting my client to the ISP modem, and then checking that website, I see NO IPv4 address and I see an IPv6 address. Before I connected my openWrt setup, I had both an IPv4 and IPv6 address. Does anyone have a guess why that is?

  2. When connecting my client to the OpenWrt modem, and then checking that website, I see an IPv4 address and a DIFFERENT IPv6 address (different from the ISP modem). I am suprised by that, I was expecting the same. Does anyone have a guess on why that is?

Thanks!

This is not OpenWrt related question.
Regarding #1 you don't have any ipv4 on the the isp modem, because it is in bridge mode. IPv6 is distributed in a different way to the devices and there are plenty of them to assign even to a bridged modem.
Regarding #2 every device has a different IPv6, so that is no surprise.

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