Subnet Mask CIDR Notation and Host Calculation
These flashcards cover subnet masks and CIDR notation — essential skills for CompTIA Network+, CCNA, and any networking course covering IP address planning. Students routinely lose marks by confusing prefix length with usable host counts or misreading broadcast addresses. These cards sharpen your ability to convert between dotted-decimal and slash notation, calculate how many hosts a subnet supports, and identify network and broadcast addresses quickly — exactly the calculations that appear on certification exams.
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5 CardsWhat is the network address of 10.0.5.37/27?
What does CIDR stand for and why was it introduced?
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How do I calculate the number of hosts in a subnet?
Use the formula 2^(32 − prefix) − 2. The 32 − prefix gives you the number of host bits. Subtract 2 to remove the network and broadcast addresses. For example, /26 = 2^6 − 2 = 62 usable hosts.
What is the difference between a subnet mask and a CIDR prefix?
Both describe the same boundary but in different formats. A subnet mask is written in dotted-decimal (e.g., 255.255.255.0), while a CIDR prefix is the count of network bits preceded by a slash (e.g., /24). They are interchangeable — 255.255.255.0 always equals /24.
Why do subnets lose 2 addresses?
Every subnet reserves its first address as the network identifier (all host bits = 0) and its last address as the broadcast (all host bits = 1). Neither can be assigned to a host. A /30 subnet, for example, has 4 total addresses but only 2 usable hosts.