Systems and methods for supporting inter subnet partitions in a high performance computing environment. In accordance with an embodiment, a fabric manager can define a range of P_Key values, among a plurality of P_Key values, as a inter subnet partition (ISP) P_Key range. The fabric manager can communicate this defined range of P_Key values to a number of subnets, via their subnet managers. The subnet managers in each subnet retain management over their subnets. As there is no central management that configures each side of inter subnet communication, subnet managers on within participating subnets can set up ISP membership, and then exchange information with the other subnet.