What role does the subnet mask play in IPv4 routing decisions?

• A. Determines the next hop by using the IP header alone.
• B. It helps determine whether a destination is on-link or requires routing via a gateway.
• C. Sets the maximum transmission unit for the path.
• D. Specifies the time-to-live value for the packet.

Answer: (B)

The subnet mask defines which addresses are considered part of the same local network. In routing decisions, a device applies the mask to the destination address to see if it falls within the same subnet as one of its own interfaces. If the destination is on the same local network, the packet can be delivered directly on that network (ARP is used to reach the destination’s MAC). If it’s not on-link, the packet must be sent to a gateway (next hop) as indicated by the routing table.

For example, with a 192.168.1.0/24 network on an interface, a packet destined for 192.168.1.55 is on-link and can be delivered directly, while a packet for 192.168.2.10 is off-link and should be forwarded to the gateway.

The subnet mask doesn’t set MTU or TTL, and routing decisions aren’t made from the IP header alone; they rely on comparing the destination address to the local subnets and consulting the routing table to choose the next hop.