An IPv4 address is 32 bits written as four decimal octets, such as 192.168.10.25. A subnet mask, such as 255.255.255.0 or /24 in CIDR notation, marks which bits are the network portion and which identify the host. Hosts with the same network portion are on the same subnet and talk directly using ARP and switching; anything else goes to the default gateway. Almost every troubleshooting question eventually depends on reading an address and mask correctly, so this is one of the most important skills for the exam.
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