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CCNA · 200-301Intermediate

IPv4 VLSM: Overlapping Subnets

A new /27 was allocated inside an existing /26 range, so two connected subnets overlap. Diagnose it and fix it yourself, live, right below.

Interactive simIPv4SubnettingVLSM

Symptom

A new subnet is added for growth but routing behaves unpredictably. Calculate the correct VLSM boundary and remove the overlap.

Your mission

Prove the cause, then fix it

Format

Guided CLI investigation

Start interactive labSee the investigation plan ↓Need a hint? Reveal the root cause ↓
IPv4 VLSM: Overlapping Subnets

Investigate before you configure.

Use the CLI evidence to isolate the fault, make the smallest safe correction, then verify the network state changed.

Observe the symptom and link state.

Diagnose by comparing the protocol evidence.

Verify the expected device state and confirm the original symptom is resolved.

Console access: R1

This workspace stacks for portrait phones. Rotate to landscape for the full split-screen console.

Need a hint?

Reveal the root cause when you're ready.

Try the investigation first, then use this as your escape hatch, not a dead end.

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Lab debrief

Turn the session into a repeatable troubleshooting pattern.

Use this reference after your attempt: first explain the symptom, then verify the evidence, then confirm the repair.

The problem

A new /27 was allocated inside an existing /26 range. Hosts on the two segments have inconsistent reachability because the address plan overlaps.

What you'll practice

  • Calculate /26 and /27 boundaries
  • Identify overlapping IPv4 ranges
  • Interpret interface addressing
  • Apply a corrected address plan
Plan the VLSM boundaries yourself — free Subnet Workbench

The topology

A router connects two LANs that should be distinct VLSM subnets but overlap due to an incorrect network boundary.

Commands to run yourself

The real diagnostic commands for this lab, copy-paste ready. Run these in the same order to reproduce the investigation on your own session.

Inspect interface addresses
show ip interface brief
Inspect connected routes
show ip route connected
IPv4 VLSM: overlapping subnetsGi0/0.20 was carved from inside the /26 already owned by Gi0/0.10.R1Gi0/0.10 + Gi0/0.20LAN10 · Gi0/0.10192.168.1.0/26 · .1-.62LAN20 · Gi0/0.20 (overlap)192.168.1.33 in .32/27 · .33-.62Address planCompare ranges

Frequently asked

Why does show ip route connected reveal the overlap when the interfaces are both up?

Both subinterfaces come up fine because each has a valid address and mask on its own. The overlap only becomes visible when you compare the two connected prefixes side by side: one range sits entirely inside the other, so the router treats part of LAN20's traffic as though it belonged to LAN10.

Ready to run this lab yourself?

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