A Stub router, One-armed router or router on a stick is a router that routes traffic between virtual local area networks (VLANs). It has only a single Ethernet NIC that is part of two or more Virtual LANs, enabling them to be joined.
A VLAN allows multiple virtual LANs to coexist on the same physical LAN. This means that two machines attached to the same switch cannot send Ethernet frames to each other even though they pass over the same wires. If they need to communicate, then a router must be placed between the two VLANs to forward packets, just as if the two LANs were physically isolated. The only difference is that the router in question may contain only a single Ethernet NIC that is part of both VLANs. Hence, "one-armed".