My domain name have no IP address when I registered it
Domain name is just a name you registered with extension, something like you have written a book and selected a name, now you want to publish it, and you need to find a publisher to do the printing and publish using the name of your book.
Just like your book, after you register a domain name, you need to find a hosting company to publish it on the internet using Internet protocol numbers, so your hosting company will provide you an IP, most of the registrars provide hosting services as well.