What happens when there are two same IP addresses?
ISPs asign one IPv4 address to a customer, and your "WiFi box" acts as a router/NAT box/ DHCP server doling out addresses from a private range internally. It is possible that the "outside" IPv4 address is itself in a private range, leaving your ISP through a mere handful  of IPv4 addresses via NAT (real one, your box really does PAT, port forwarding through one IPv4 address).