Nowadays, peer to peer systems are largely studied. But in order to evaluate them in a realistic way, a better knowledge of their environments is needed. In this article we focus on the computers availability in these systems. We characterize this availability behind ADSL lines and we link it with the availability of peer to peer systems participants. We emphasise on the methodology as generalized in other systems such as grids or ad-hoc systems. We finally show how users of ADSL lines are related to peer to peer users and we give some examples of the possible practical use of theses results. The results are based on trace datasets obtained over the first five month of 2003 with around 5000 hosts.