Advanced lesson
Subnets and VLANs: Build a Small Address Plan
Work through a /26 example, understand the gateway and separate addressing from access control.
Reviewed September 2026 · 3 minute read
Split one /24 into four /26 networks
A prefix tells you how many address bits describe the network. IPv4 has 32 bits. A /26 leaves 6 host bits: 2⁶ = 64 addresses. For an ordinary broadcast subnet, the network and broadcast addresses leave 62 assignable host addresses. /31 and /32 designs have different uses and are outside this exercise.
| Network | Usable host addresses | Broadcast |
|---|---|---|
| 192.168.20.0/26 | .1–.62 | .63 |
| 192.168.20.64/26 | .65–.126 | .127 |
| 192.168.20.128/26 | .129–.190 | .191 |
| 192.168.20.192/26 | .193–.254 | .255 |
The mask is 255.255.255.192. Each block advances by 64. Include the gateway and infrastructure when counting required addresses; not all 62 are available to end users.
A plan for staff and guests
Worked example
Thirty staff devices and a separate guest area
| Purpose | Subnet | Gateway | Example DHCP pool |
|---|---|---|---|
| Staff / VLAN 20 | 192.168.20.0/26 | 192.168.20.1 | 192.168.20.10–50 |
| Guests / VLAN 30 | 192.168.20.64/26 | 192.168.20.65 | 192.168.20.70–120 |
These are lab values. Check overlap with existing LANs, VPNs and routed networks before using any real address plan. Keep gateway, reserved addresses and DHCP pools documented.
Separate addresses still need an access policy
A VLAN creates a separate Layer 2 network on suitable switching equipment. Devices in different subnets need routing to communicate. Whether that communication is allowed depends on the applicable policy and where it is enforced.
An endpoint port commonly assigns untagged traffic to one VLAN. A link between network devices can carry several tagged VLANs. Both ends must agree. “Trunk” describes this kind of multi-VLAN link; it does not mean unlimited access.
IPv6 does not use the IPv4 broadcast/host-count rule above. IPv6 addressing and rules need their own design and checks.
Practice before touching a switch
Your exercise needs 35 client addresses, one gateway and four infrastructure addresses, with growth expected. Explain why a /27 is too small and why a /26 deserves consideration. Then identify the gateway, pool and reserved range in your proposal.
Show the reasoning
An ordinary /27 provides 30 assignable hosts, below the 40 already required. A /26 provides 62, leaving 22 before further reservations. Check growth expectations before choosing it. Capacity alone does not decide access policy.
Put it into practice
Check what you learned
Choose an answer for each question, then check your understanding. Your answers stay in this browser.
Read the answers and explanations
- 62
There are 64 total addresses; this ordinary subnet reserves network and broadcast addresses.
- 192.168.20.90
The network spans .64 through .127; usable hosts are .65 through .126.
- The required allowed and blocked access, tested from each group.
Separate addressing does not by itself prove isolation.
Sources and further reading
- IETF RFC 1918: private IPv4 addresses
The primary reference for the private address ranges used in our examples.
- IETF RFC 8200: IPv6
The protocol reference behind IPv6; use it for deeper study after the short lesson.
Keep your momentum
Ready for the next step?
If you can explain the example and your answer, try the next lesson. Revisit the feedback whenever you need it.
FryAVSI · Updated September 10, 2026
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FryAVSI · Northeast Ohio and beyond
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