Two PCs have addresses 192.168.25.128/25 and 192.168.25.100/25. Which subnet mask should both use so they are on the same local network?

The correct subnet mask is 255.255.255.0 because it places both IP addresses, 192.168.25.128 and 192.168.25.100, within the same subnet. A /24 (255.255.255.0) mask means the first three octets (192.168.25) define the network portion, and the last octet defines the host portion. Both addresses share the same network portion (192.168.25), making them part of the same local network. The other options (255.255.255.248, 255.255.255.224, 255.255.255.252) are smaller subnets (e.g., /29, /27, /30 respectively). With these masks, 192.168.25.128 and 192.168.25.100 would fall into different subnets, preventing direct communication without a router. For example, with 255.255.255.224 (/27), 192.168.25.100 is in the 192.168.25.96 network, while 192.168.25.128 is in the 192.168.25.128 network.