为什么IPv4地址用完了？

I understand that we are running out (or ran out already?) of IPv4 addresses, but I don't really understand why that is. Right now, every home has its own IP address (dynamically assigned, but still, each has an address). Why can't a city (for example) have just one IP address and all homes in this city would just be on a private network of that city? Then this one city would be able to assign addresses from range 0.0.0.1 to 255.255.255.254.I understand that we are running out (or ran ou