Recently, various proposals for supporting network mobility, which provides efficient Internet access when a network formed within a vehicle moves around as a unit, have emerged. The schemes in those proposals, though, manifest some major drawbacks with respect to scalability: If the number of mobile nodes within a mobile network is large, the handoff latency would increase greatly, causing communication disruption; Data delivery to a node within a nested mobile network nay suffer extremely inefficient pinball routing. We propose a scalable network mobility supporting mechanism named SNEMOS (Scalable NEtwork Mobility Support), which resolves the above two major problems of the existing schemes. The performance of SNEMOS is compared with the existing schemes through extensive simulations. The numerical results show that SNEMOS outperforms the existing schemes with respect to handoff latency hop counts of routing paths, packet delivery time, header overhead in data packets, and signaling overhead.