You can very easy end up with multilevel nat. The last mile (ie a apartment complex) can have one and then the customer router is the second.
But that assumes that the ISP has addresses to spare. If not, we have the three layers of nat. ISP->apartment complex->customer router->all customer devices.
And then we have intermediates, like ISP->municipal->apartment complex->customer router->customer devices. Now we have four layers of nat.
In the really worse case, ISP are split up into multiple ones, and you can get carrier grade NAT in the form of ISP->ISP->municipal->apartment complex->customer router->customer devices. Five layers of nat and up.
Multilevel NAT is a form of sadness that NAT never was designed to do.
But that assumes that the ISP has addresses to spare. If not, we have the three layers of nat. ISP->apartment complex->customer router->all customer devices.
And then we have intermediates, like ISP->municipal->apartment complex->customer router->customer devices. Now we have four layers of nat.
In the really worse case, ISP are split up into multiple ones, and you can get carrier grade NAT in the form of ISP->ISP->municipal->apartment complex->customer router->customer devices. Five layers of nat and up.
Multilevel NAT is a form of sadness that NAT never was designed to do.