What is the primary function of a router in a computer network?

Network devices are classified by the OSI layer they operate at and the specific job they perform. A device operating at the data-link layer (Layer 2) forwards frames between ports using MAC addresses within one network -- this is switching. A device operating at the network layer (Layer 3) forwards packets between separate networks using IP addresses and a routing table to select the best path -- this is routing. A device that inspects traffic against a configured rule set to permit or block it -- typically by IP address, port, or protocol -- is performing security/policy filtering, most commonly associated with a firewall or an access-control list.

The question asks for the primary, defining function of a router. Interconnecting separate networks and forwarding data between them based on destination IP addresses is exactly the Layer-3 routing job described above, so "To connect multiple networks together" captures the router's defining role.

Checking this against each of the other listed descriptions:

• "To connect devices within the same network" describes Layer-2 frame forwarding by MAC address inside one broadcast domain -- a distinct job from forwarding between separate networks.

• "To filter incoming traffic based on IP addresses" describes a policy-enforcement action -- permitting or dropping packets against a configured rule set. A router can indeed be configured to do this through an access-control list, but that is an optional capability added on top of forwarding; it is the defining job of a firewall, not of the device the question asks about.

• "More than one of the above" would require at least two of the listed descriptions to each independently capture the primary, defining role. Only the Layer-3 inter-network forwarding description does so: Layer-2 switching within one network is a different device's defining job, and IP filtering is a configured extra rather than a defining role.

• "None of the above" would require that not one of the listed descriptions names the primary function the question asks about. The Layer-3 description -- forwarding packets between separate networks using destination IP addresses -- does name it, so that condition is not met.

Cross-checking against the OSI model confirms this: any device that makes forwarding decisions purely from a Layer-3 destination IP address, across separate broadcast domains, is by definition performing routing -- confirming that connecting multiple networks together is the router's primary function. This also matches the official answer key for the BPSC TRE 3.0 (Class 11-12) Computer Science paper held on 22 July 2024, from which this question is taken.