An IPv4 address is 32 bits written as four decimal octets, such as 192.168.10.37. The subnet mask says how many of those bits identify the network and how many identify the host. In prefix notation, /24 means the first 24 bits are network bits, which is the mask 255.255.255.0. Subnetting is the skill of borrowing host bits to make more, smaller networks. The CCNA expects you to do it quickly and accurately without a calculator, because subnetting steps hide inside routing, ACL (access control list) and troubleshooting questions.
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