To support mobility, multihoming, routing scalability, and security, there are a lot of proposals based on ID/Locator split approach not only for the current Internet but also for the future Internet. However, none of them meet the requirements for practical operation such as (1) support of heterogeneity of network layer protocols, (2) scalability of ID/Locator mapping system, (3) independence of mapping information management, and (4) avoidance of locator leakage beyond the administrative boundary. This paper proposes a network layer protocol called Z Network Protocol (ZNP) for the future Internet based on clean slate approach. ZNP supports heterogeneity of network layer protocols by "Internetworking with a Common ID Space". Its mapping system meets the requirements (2)-(4) described above. Z Control Message Protocol (ZCMP) is also designed. Currently, implementation of ZNP and ZCMP is ongoing.