CIDR subnetting for CCNA: /26, /27, /28 explained
CIDR notation (Classless Inter-Domain Routing) is the standard way to express subnet masks on modern networks. The number after the slash indicates how many bits are used for the network portion; the remaining bits are for hosts. On the CCNA exam, you will encounter /26, /27, and /28 subnets frequently. Understanding how to calculate host counts, usable addresses, and subnet boundaries for these three masks is essential for both the exam and real-world deployment.
The /26 subnet mask: 64 addresses per subnet
A /26 subnet uses 26 bits for the network and leaves 6 bits for hosts. With 6 host bits, you can address 2 to the power of 6 = 64 total addresses per subnet. In decimal, /26 equals 255.255.255.192.
Of those 64 addresses, the first is the network address and the last is the broadcast address, leaving 62 usable host addresses. This makes /26 a common choice for small office networks or point-to-point links that need more than a /27 or /28 can offer.
Example: 192.168.1.0/26 Network address: 192.168.1.0 First usable IP: 192.168.1.1 Last usable IP: 192.168.1.62 Broadcast address: 192.168.1.63 Next subnet: 192.168.1.64
The /27 subnet mask: 32 addresses per subnet
A /27 subnet uses 27 bits for the network and 5 bits for hosts. This yields 2 to the power of 5 = 32 total addresses per subnet. The decimal equivalent is 255.255.255.224. You lose 2 addresses (network and broadcast), leaving 30 usable host IPs.
/27 is frequently used in enterprise environments where you need to maximize the number of subnets from a given address space while still accommodating small to medium departments or VLANs. It strikes a practical balance between efficiency and usability.
Example: 10.0.0.0/27 Network address: 10.0.0.0 First usable IP: 10.0.0.1 Last usable IP: 10.0.0.30 Broadcast address: 10.0.0.31 Next subnet: 10.0.0.32
The /28 subnet mask: 16 addresses per subnet
A /28 subnet uses 28 bits for the network and 4 bits for hosts. This produces 2 to the power of 4 = 16 total addresses per subnet, or 255.255.255.240 in dotted decimal. After removing network and broadcast addresses, you have 14 usable IPs.
/28 is ideal for point-to-point links, small branch offices, or IoT device subnets where you need many subnets but few hosts per subnet. On the CCNA exam, /28 questions often test your ability to calculate multiple subnets quickly from a larger block.
Example: 172.16.0.0/28 Network address: 172.16.0.0 First usable IP: 172.16.0.1 Last usable IP: 172.16.0.14 Broadcast address: 172.16.0.15 Next subnet: 172.16.0.16
Quick reference and exam tips
- →/26: 64 addresses, 62 usable, subnet increment of 64
- →/27: 32 addresses, 30 usable, subnet increment of 32
- →/28: 16 addresses, 14 usable, subnet increment of 16
- →Always subtract 2 from the total address count for network and broadcast
- →Subnet increment equals the total address count (64, 32, or 16)
- →On exams, practice identifying which mask fits a given requirement
- →Use the formula: 2^(32 - CIDR) to find total addresses in any IPv4 subnet
Mastering these three masks will accelerate your subnetting speed during the CCNA exam. The key is to memorize the address counts and practice identifying subnet boundaries under time pressure. Working through dozens of examples will build the muscle memory you need.