The rapid growth of IP-based mobile telecommunication technologies in the past few years has revealed situations where not only a single node but an entire network moves and changes its point of attachment to the Internet. The main goal of any protocol supporting network mobility (NEMO) is to provide continuous, optimal and secure Internet access to all nodes and even recursively nested mobile subnetworks inside a moving network. This paper describes a Host Identity Protocol (HIP) extension called HIP-NEMO, based on hierarchical topology, signaling delegation and connection tracking to enable secure and efficient network mobility support in the HIP layer.