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VLSM 43 — 120 / 58 / 30 hosts
Enterprise Intermediate 4 devices 4 checked tasks 100 points Plan lab
Cut 172.26.42.0/24 into three right-sized subnets for 120, 58 and 30 hosts (largest first: /25, /26, /27), and give the router the first usable address of each. The PCs are already addressed; make them reach each other. You build it in a browser, type real commands at each device, and every task below is
checked against the running configuration — so you find out it is wrong before anybody else does.
Open this lab in the simulator
The simulator runs in your browser with nothing to install. This lab is part of the full simulator: open it free for 7 days with a new account, then with a plan. It needs an account, which keeps your progress.
The scenario
Cut 172.26.42.0/24 into three right-sized subnets for 120, 58 and 30 hosts (largest first: /25, /26, /27), and give the router the first usable address of each. The PCs are already addressed; make them reach each other.
Your role: a network engineer on the enterprise team. Level: Intermediate Done when: all 4 goals show as passed (100 points).
Network status
Devices: 1 router, 3 PCs, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. HR (120 hosts) is a host with 172.26.42.2/25, gateway 172.26.42.1. Guests (58 hosts) is a host with 172.26.42.130/26, gateway 172.26.42.129. Faculty (30 hosts) is a host with 172.26.42.194/27, gateway 172.26.42.193. R-Aizawl starts with an empty configuration. Known problems
None: nothing is misconfigured. The network is simply not finished: what the goals list is what you have to build or secure. Goals
HR: 120 hosts → /25; Gi0/0 = 172.26.42.1 (20 pts) Guests: 58 hosts → /26; Gi0/1 = 172.26.42.129 (20 pts) Faculty: 30 hosts → /27; Gi0/2 = 172.26.42.193 (20 pts) All three networks reach each other (40 pts) The activity, in order
HR: 120 hosts → /25; Gi0/0 = 172.26.42.1 Why: A router interface routes nothing until it has an address and is up. Every other task builds on this one. Verify with: show ip interface brief Guests: 58 hosts → /26; Gi0/1 = 172.26.42.129 Why: A router interface routes nothing until it has an address and is up. Every other task builds on this one. Verify with: show ip interface brief Faculty: 30 hosts → /27; Gi0/2 = 172.26.42.193 Why: A router interface routes nothing until it has an address and is up. Every other task builds on this one. Verify with: show ip interface brief All three networks reach each other Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 172.26.42.130 Tips
Work in the order given: later goals depend on the earlier ones. After every change, run the verify command before moving on; do not guess. Stuck? Read the verify output first. It names the interface, neighbour or route that is wrong. Common mistake: Forgetting the network and broadcast addresses when counting hosts Common mistake: Allocating small subnets first and misaligning the big one Common mistake: Giving the router the network address instead of the first host Why this matters
Giving every LAN a /24 wastes most of the address space. VLSM sizes each subnet to its need: the number of host bits is the smallest n with 2ⁿ − 2 ≥ hosts, and the largest subnet is allocated first so the blocks stay aligned.
What you will be able to do
Work out the prefix length for a host requirement
Allocate subnets in descending order
Address interfaces from the plan
The network
ISR 2911, PC — 4 devices in all, already cabled.
Fa0 Gi0/0 Fa0 Gi0/1 Fa0 Gi0/2 R-Aizawl HR (120 hosts) 172.26.42.2/25 · gw 172.26.42.1 Guests (58 hosts) 172.26.42.130/26 · gw 172.26.42.129 Faculty (30 hosts) 172.26.42.194/27 · gw 172.26.42.193 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
R-Aizawl ISR 2911 configure it HR (120 hosts) PC 172.26.42.2 / 255.255.255.128 gw 172.26.42.1 Guests (58 hosts) PC 172.26.42.130 / 255.255.255.192 gw 172.26.42.129 Faculty (30 hosts) PC 172.26.42.194 / 255.255.255.224 gw 172.26.42.193
Cabling
HR (120 hosts) Fa0 ↔ R-Aizawl Gi0/0 Guests (58 hosts) Fa0 ↔ R-Aizawl Gi0/1 Faculty (30 hosts) Fa0 ↔ R-Aizawl Gi0/2
What is configured at the start
Nothing is configured to start with beyond the addresses above: you build the configuration from a clean start.
Tasks, and how each one is checked
Every task is decided by reading the live network — a neighbour in FULL, a ping that succeeds, a line in the running configuration — not by looking for a keyword you typed. The same command shows you what the lab sees.
# Task Pts How to verify it yourself
1 HR: 120 hosts → /25; Gi0/0 = 172.26.42.1 20 On R-Aizawl: show ip interface brief — Gi0/0 has 172.26.42.1 and is up/up. 2 Guests: 58 hosts → /26; Gi0/1 = 172.26.42.129 20 On R-Aizawl: show ip interface brief — Gi0/1 has 172.26.42.129 and is up/up. 3 Faculty: 30 hosts → /27; Gi0/2 = 172.26.42.193 20 On R-Aizawl: show ip interface brief — Gi0/2 has 172.26.42.193 and is up/up. 4 All three networks reach each other 40 On HR (120 hosts): ping 172.26.42.130 must succeed. and On Guests (58 hosts): ping 172.26.42.194 must succeed.
Stuck? Ask the coach
Tell the coach what you see or what you tried. It gives you a hint, not the answer, the way a senior engineer would. Included with a free trial or any plan.
Ask the coach
Common mistakes
Forgetting the network and broadcast addresses when counting hosts
Allocating small subnets first and misaligning the big one
Giving the router the network address instead of the first host
Reference solution
Try it yourself first — the lab checks your work either way. This is one configuration that makes every task pass, device by device:
R-Aizawl interface g0/0
ip address 172.26.42.1 255.255.255.128
no shutdown
exit
interface g0/1
ip address 172.26.42.129 255.255.255.192
no shutdown
exit
interface g0/2
ip address 172.26.42.193 255.255.255.224
no shutdown
exit Where this is examined
CCNA 1.0 Network Fundamentals (IPv4 addressing); Network+ 1.4.
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