On PMIPv6-Based Mobility Support for Hierarchical P2P-SIP Architecture in Intelligent Transportation System Network Service providers have many worries about providing network services with an expandable, reliable, flexible and low-cost structure according to the expanding market environment. The current client-server system has various problems such as complexity and high costs in providing networkservices. On the contrary to this, this problem can be simply solved, if the Peer-to-Peer (P2P) communication terminal supporting access of distributed resources provides functions which the current Session Initiation Protocol (SIP) -based network devices have. Because diverse terminals in anetwork access through networks, also, partitioning network domains with gateways to manage, and applying the Proxy Mobile IPv6 (PMIPv6) technology considering mobility of terminals would help tohave a more efficient network structure. Especially, the proposed P2P-SIP structure proves itself as a very efficient structure to have an outstanding expandability among different networks in a region, andto reduce maintenance costs.