In this paper we explore various network layer concepts that pertain to the design of mobile networking systems. We show that mobility is essentially an {\em address translation} problem and is best resolved at the network layer. We have identified the fundamental services that must be supported at the network layer to carry out the task of address translation. Using these service primitives as building blocks, we propose a network layer architecture which enables smooth integration of mobile end systems within the existing Internet. The architecture is modularized into well-defined logical components. In this paper our objective is not to propose {\em a specific scheme} for supporting mobility, rather it is to highlight and analyze the essential aspects of supporting mobile end-systems, as well as to better understand the trade-off between various design alternatives. (Also cross-referenced as UMIACS-TR-95-117)