Carrier Sense Multiple Access / ID Countdown (CSMA/IC) is a collision-free ad hoc MAC protocol. CSMA/IC avoids simultaneous medium access by competing with a local unique ID assigned to each node. However, assigning the local unique ID to each node is a very complicated problem. In this paper, we propose a simple distributed ID assignment scheme for CSMA/IC. Investigating CSMA/IC, we found that the largest ID screens all IDs of contenders during the contention process. Due to this “ID screen effect”, every node can estimate the largest ID among its contenders. Using this knowledge, we propose a self unique ID assignment scheme which does not require message exchanges. To complement our ID assignment scheme, we also suggest a “well arranged rotation ID system”. The proposed ID assignment scheme suffers from an ID increasing problem and an unfair medium access problem. However, the new ID system mitigates these problems in a very simple way. Simulation results show the proposed ...