A subinterface is a logical interface created on a single physical router interface that allows routing between VLANs (known as Router-on-a-Stick (ROAS)). This method is commonly used when only one physical router is available, allowing inter-VLAN communication without additional hardware. *Why not the other options? *VXLAN (B) - This is used for extending Layer 2 networks over a Layer 3 infrastructure, primarily in data centers. It does not directly enable inter-VLAN communication. *Layer 3 switch (C) - A Layer 3 switch can route between VLANs, but the scenario states that purchasing additional hardware is not an option. *VIR (D) - This is not a standard networking term in the context of VLAN communication. Reference: CompTIA Network+ (N10-009) Official Guide - Chapter 8: VLANs and Inter-VLAN Routing