Peer-to-Peer (P2P) networks work on the presumption that all nodes in the network are connectable. However, NAT boxes and firewalls prevent connections to many nodes on the Internet. For UDP based protocols, the UDP hole-punching technique has been proposed to mitigate this problem.This paper presents a study of the efficacy of UDP hole punching on the Internet in the context of an actual P2P network. To the best of our knowledge, no previous study has provided similar measurements. Our results show that UDP hole punching is an effective method to increase the connectability of peers on the Internet: approximately 64% of all peers are behind a NAT box or firewall which should allow hole punching to work, and more than 80% of hole punching attempts between these peers succeed.