Monaco are currently top of Ligue 1 and have made an unbeaten start to the season but Ranieri believes that his side need to focus on improving at the back ahead of the visit of Saint-Etienne.
"I want the team to be more compact, always," the Italian told reporters.
"Saturday comes another good team that plays good football, that plays well on the sides, so there are a lot of players who dribble very well, very quickly. It is important to be focused and play compact.
"We have many young players so it's normal that we only think of the attack but, in football, there is the attack and defence."
Ranieri guided Monaco to promotion by winning Ligue 2 last season.