A network administrator notices that devices within the same subnet (192.168.10.0/24) are unable to communicate directly with each other and instead…

An end device's operating system uses the subnet mask to perform a critical calculation: it determines whether a destination IP address is on the local network or a remote network. It does this by comparing the network portion of its own address with the network portion of the destination address. If the subnet mask is misconfigured to be more restrictive than it should be (e.g., 255.255.255.240 or /28 instead of the correct 255.255.255.0 or /24), the device will incorrectly conclude that other devices on the same physical segment are on a different, remote network. Consequently, instead of sending an ARP request to find the destination's MAC address for direct communication, it will forward the packet to its configured default gateway for routing.

A. Incorrect DNS server configurations would affect the resolution of domain names to IP addresses, but not the IP routing logic for traffic between two known IP addresses.

B. A switch operating at Layer 3 functions as a router. However, it does not force devices on the same subnet/VLAN to route through it; the end devices themselves must decide to send traffic there.

C. A default gateway is a passive device in the routing process; it only forwards packets that are explicitly sent to it. It cannot "override" or intercept traffic between two hosts on the same local network.

1. Kurose, J. F., & Ross, K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson. In Section 4.4.1, "The Forwarding Table," the text describes how a host's forwarding logic determines where to send a packet. A host's forwarding table typically has a rule for its directly attached network and a default route. A misconfigured subnet mask causes the host to fail the check for the directly attached network, forcing it to use the default route (the default gateway).

2. Stallings, W. (2014). Data and Computer Communications (10th ed.). Pearson. Chapter 18, "Internet Protocol," in the section on "IP Addresses," explains that the subnet mask is used to extract the network ID from an IP address. The host uses this to determine if a destination is on its own network. An incorrect mask leads to an incorrect network ID calculation, causing the host to forward the packet to a router.

3. Odom, W. (2013). Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide. Cisco Press. Chapter 16, "Fundamentals of IP Addressing and Routing," details the IP routing process for a host. It explicitly states that the host performs a logical comparison using its own IP/mask and the destination IP/mask. If the resulting network IDs do not match, the packet is sent to the default gateway. This process is fundamentally dependent on a correctly configured subnet mask.