This paper presents an overview of MPICH-GP, which extends the MPICH-G2 functionality to support private IP clusters. To support the communication among private IP clusters, the MPICH-GP uses a communication relay scheme combining the NAT and a user-level proxy. The benchmarking results show that MPICH-GP outperforms the user-level twoproxy scheme used in PACX-MPI and Firewall-enabled MPICH-G, and also show comparable application performance to that of original MPICH-G2, especially in large message (or problem) size.