Subnetting made simple for Network Plus
Memorizing a table of "slash values to host counts" is the slow road to subnetting. You forget half of it under exam pressure and the question changes the numbers on you anyway. There is a method that runs in your head in under thirty seconds, scales to any prefix, and survives the worked examples on Network Plus N10-009 objective 1.4. This is that method.
What problem does subnetting actually solve
Subnetting takes one big IP range and slices it into smaller, isolated networks. Each slice gets its own broadcast domain, its own routing, and usually its own VLAN. Network Plus expects you to size a slice to fit a host count, write out the network address, the broadcast address, and the usable host range. That is the entire game on the exam.
The two foundational RFCs are RFC 1918, which carves out the private address space (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) that lives inside every corporate network, and RFC 950, which originally introduced the subnet mask concept.
The block-counting method, end to end
Forget tables. Use the block-counting method and you can derive every subnet attribute live, without memorization. Three rules carry the whole technique.
- Rule 1: Block size equals 256 minus the mask octet. For /26, the mask is 255.255.255.192. The interesting octet is 192. The block size is 256 minus 192, which equals 64. Subnets step by 64 in that octet.
- Rule 2: 2^n total addresses, minus 2 usable. n is the number of host bits left. /26 leaves 6 host bits. 2^6 equals 64 total addresses per subnet. Subtract 2 (network and broadcast) and you get 62 usable hosts.
- Rule 3: The first address is the network, the last is the broadcast. Everything between them is usable. The first usable IP is network plus 1. The last usable IP is broadcast minus 1.
Combine the three and any subnet question collapses to arithmetic you can do in your head.
A worked example, end to end
You manage 192.168.1.0/24 and your boss says "split it into four equal subnets for a 30-user office that needs guest Wi-Fi on its own slice." Walk it.
Given: 192.168.1.0/24 (256 addresses, mask 255.255.255.0) Want: 4 equal subnets Step 1: 4 subnets = 2 borrowed bits. /24 + 2 = /26. Step 2: New mask = 255.255.255.192. Block size = 256 - 192 = 64. Step 3: Subnets step by 64 in the last octet. Subnet 1: 192.168.1.0/26 Network: 192.168.1.0 Usable: 192.168.1.1 - 192.168.1.62 Broadcast: 192.168.1.63 Subnet 2: 192.168.1.64/26 Network: 192.168.1.64 Usable: 192.168.1.65 - 192.168.1.126 Broadcast: 192.168.1.127 Subnet 3: 192.168.1.128/26 Network: 192.168.1.128 Usable: 192.168.1.129 - 192.168.1.190 Broadcast: 192.168.1.191 Subnet 4: 192.168.1.192/26 Network: 192.168.1.192 Usable: 192.168.1.193 - 192.168.1.254 Broadcast: 192.168.1.255 Hosts per subnet: 2^6 - 2 = 62. More than enough for 30 users.
Notice what happened. You never looked at a chart. You did not memorize that /26 equals 62 hosts. You derived it. The block size told you where the subnets started, the 2^n minus 2 rule told you how many hosts fit, and the boundaries fell out.
CIDR shorthand and what to recognize at a glance
Network Plus uses CIDR notation everywhere, so you should recognize the common ones without thinking.
- /24. 256 addresses, 254 usable. The "classic" small office subnet.
- /25. 128 addresses, 126 usable. Used for splitting a /24 in half.
- /26. 64 addresses, 62 usable. Often used for guest networks.
- /27. 32 addresses, 30 usable. Common for small departments or a VoIP VLAN.
- /28. 16 addresses, 14 usable. A handful of servers, a DMZ, a management VLAN.
- /30. 4 addresses, 2 usable. Point-to-point links between routers.
- /31. 2 addresses, 2 usable (RFC 3021 allowed this for point-to-point, with no network or broadcast reserved).
Real-world: a 30-user office with guest Wi-Fi
A property management office in the south suburbs runs a 30-user staff network and wants guest Wi-Fi isolated. You take 192.168.1.0/24 and split it into 192.168.1.0/25 for staff (126 usable hosts, way more than enough) and 192.168.1.128/25 for guests. Two /25 subnets. Two VLANs. Inter-VLAN routing controlled by a firewall rule that allows guests to reach the internet but blocks them from staff.
If you needed four subnets (staff, guest, voice, cameras), the same /24 splits into four /26 subnets. 62 hosts per subnet. Voice and cameras typically need fewer, so they have headroom. Staff gets clean isolation. That is a real design and the exam asks the math version of it constantly.
What this looks like in our platform
The Study Mode deck for objective 1.4 has timed subnetting drills that start at /24 and walk through every common prefix. The Practice Exam mixes subnetting into multiple-choice and performance-based questions where you have to size a real network. The Net+ track page shows where this fits in the broader objective map.
When you hit a wall, drill the block size
If subnetting still feels slow, drill one thing: block size from the mask. Look at the mask octet, subtract from 256, say the block out loud. Do twenty in a row. The rest of the technique sits on top of that one skill. Once block size is automatic, everything else falls in line.
Sources
- CompTIA. CompTIA Network+ certification overview. Exam code N10-009. Domain weights and objective list.
- IETF. RFC 1918: Address Allocation for Private Internets. Defines the 10/8, 172.16/12, and 192.168/16 private ranges.
- IETF. RFC 950: Internet Standard Subnetting Procedure. The original subnet mask specification.
About the authors
IT Service Center Manager and former CTE / IT teacher. Owner of Revtek IT Solutions. Writes everything that ships under his name and reviews every line of Revy-assisted drafting before publish.
LinkedIn ↗Revy helps draft and structure these posts. Every piece is reviewed, edited, and fact-checked by Nick before publish. We disclose this here because it is the right thing to do. See the AI Policy for the full stance.