For five isolated networks within a building, each with its own devices, connected to a single ISP, how many NICs are required on the router, and how many switches are needed?

• A. Five NICs and five switches
• B. Six NICs for the router and four switches
• C. Seven NICs and six switches
• D. Six NICs for the router and five switches

Answer: (D)

When you have multiple isolated LANs that all share one Internet connection, each LAN needs its own path to the router so traffic can be kept separate and routed appropriately. That means five router interfaces dedicated to the five LANs. In addition, the router must have a connection to the Internet via the ISP, which requires one more interface. So the router needs six NICs in total: five for the internal networks and one for the WAN to the ISP.

For the switches, each isolated network should have its own switch to provide Ethernet access for the devices in that network and to connect upward to its dedicated router interface. With five separate networks, you’d typically have five switches—one per network. (If you used VLANs on a single switch, you could potentially reduce the number of switches, but the described setup calls for five switches.)