Casillas has signed a new 12-month deal with Porto.
"In a year's time, we'll see where we are. A big part of my decision to stay on was down to the fans. They gave me a lot of support," he said of Porto.
“I can see myself going back to Real Madrid one day. Whenever I am home I go to the Bernabéu to cheer the team on. I don't know when I will return to the club. I started out there as a kid, I grew up there.
"I spent so much of my life there that everyone saw me grow up into a man. I'm still associated with Madrid. I'm sure I'll go back. How can you say no to Real Madrid? But right now I don't see myself taking up coaching or a backroom post."