Each internet service provider owns a set of IP addresses. IPs are automatically allocated to devices using DHCP, dynamic host configuration protocol. The device tries to connect and is given one of your ISPs addresses to which it holds onto for a period of time. It can refresh the IP and keep it for longer if required. If everyone was able to request their own IPs there would be multiple devices with the same IPs. This is fine in two seperate networks that are private behind a router using NAT, like in your house. NAT basically takes a public internet IP address which is given to your router with DHCP by your ISP and splits it up using ports. So effectively even though you have ten devices behind your router such as laptops, mobiles, fridges all with private IP addresses, you actually only have a single public IP address provided to you by your ISP.

Note; I'm currently working and typed on my mobile so apologies if I missed out anything. Without delivering a CCNA course its a pretty complex subject.