In VANET, nodes move at high speed and change connecting network frequently. As a consequence, IP duplication may occur and packets would not be sent to the correct destination. This paper aims at reducing the size of location information in duplicate address detection (DAD) scheme. We propose a grid-based duplicate address detection (GDAD) scheme and separate network into several grids, where the location information in the packet is replaced with the grid number. In addition, we model and calculate the probability of occurrence of IP duplication. Finally, we simulate and estimate the probability of IP duplication.