17.8.3 Packet Tracer – Troubleshooting Challenge (Instructor Version)
Topology
17.8.3 Packet Tracer – Troubleshooting Challenge
Addressing Table
| Device | Interface | IP Address / Prefix | Default Gateway |
|---|---|---|---|
| R1 | G0/0 | 172.16.1.62 /26 | N/A |
| G0/0 | 2001:db8:cafe::1/64 | ||
| G0/0 | fe80::1 | ||
| G0/1 | 172.16.1.126 /26 | N/A | |
| G0/1 | 2001:db8:cafe:1::1/64 | ||
| G0/1 | fe80::1 | ||
| G0/2 | 172.16.1.254 /25 | N/A | |
| G0/2 | 2001:db8:cafe:2::1/64 | ||
| G0/2 | fe80::1 | ||
| S0/0/1 | 10.0.0.2 /30 | N/A | |
| S0/0/1 | 2001:db8:2::1/64 | ||
| S0/0/1 | fe80::1 | ||
| Main | S0/0/0 | 209.165.200.226 /30 | N/A |
| S0/0/0 | 2001:db8:1::1/64 | ||
| S0/0/0 | fe80::2 | ||
| S0/0/1 | 10.0.0.1 /30 | N/A | |
| S0/0/1 | 2001:db8:2::2/64 | ||
| S0/0/1 | fe80::2 | ||
| S1 | VLAN 1 | 172.16.1.61 /26 | 172.16.1.62 |
| S2 | VLAN 1 | 172.16.1.125 /26 | 172.16.1.126 |
| S3 | VLAN 1 | 172.16.1.253 /25 | 172.16.1.254 |
| IT | NIC | 172.16.1.1 /26 | 172.16.1.62 |
| 2001:db8:cafe::2/64 | fe80::1 | ||
| fe80::2 | |||
| Marketing | NIC | 172.16.1.65 /26 | 172.16.1.126 |
| 2001:db8:cafe:1::2/64 | fe80::1 | ||
| fe80::2 | |||
| R&D | NIC | 172.16.1.129 /25 | 172.16.1.254 |
| 2001:db8:cafe:2::2/64 | fe80::1 | ||
| fe80::2 | |||
| Web | NIC | 64.100.0.3 /29 | 64.100.0.1 |
| 2001:db8:acad::3/64 | fe80::1 | ||
| fe80::2 |
A note on the addressing scheme in this lab
The IP addresses in this activity are given to you — this is a troubleshooting lab, not a subnet design lab. Your job is to find and fix the misconfigured devices, not to recalculate the addressing.
That said, some students notice something and get confused, so it is worth explaining. The three subnets were allocated in the order they are listed (IT → Marketing → R&D), not in the order that standard VLSM would use. Standard VLSM always allocates the largest subnet first, which would produce a different — and equally valid — set of addresses:
| Subnet | Hosts | This lab | Standard VLSM |
|---|---|---|---|
| R&D | 100 | 172.16.1.128/25 | 172.16.1.0/25 |
| IT | 50 | 172.16.1.0/26 | 172.16.1.128/26 |
| Marketing | 50 | 172.16.1.64/26 | 172.16.1.192/26 |
Both schemes are correct: no overlaps, every block sits on a valid boundary, and both use all 256 addresses of the /24 with zero waste. If you calculated the second column on your own, you did the subnetting right — just configure the devices to match the addressing table provided in the lab. You can check either scheme with the VLSM subnet calculator, which walks through the allocation step by step in binary.
So why does VLSM insist on largest-first? Because it only works out this cleanly by coincidence here. Reorder the allocation to IT (/26) → R&D (/25) → Marketing (/26) and the pointer lands on 172.16.1.64, which is not a valid /25 boundary. You would be forced to skip forward to 172.16.1.128 and strand 64 addresses. Allocating the largest block first guarantees the smaller blocks always fit into what is left.
Objectives
• Troubleshoot common issues in a LAN.
Background / Scenario
After an update to the network, some devices were misconfigured. You must correct the configurations and verify that all the PCs can access the Web server, R1, and the switches. In addition, all PCs should be able to access R1 using SSH. You should verify connectivity for both IPv4 and IPv6 except for the switches.
Instructions
Router R1 and all the switches have been preconfigured with the following:
• Enable password: Ciscoenpa55
• Console password: Ciscoconpa55
• Admin username and password for SSH: Admin1/Admin1pa55
Required number of hosts per subnet:
• IT: 50 hosts
• Marketing: 50 hosts
• R&D: 100 hosts
If all configuration problems have been fixed, all devices should be able to ping each other and the web server.
Issues Found and Solutions
| # | Device | Issue | Solution |
|---|---|---|---|
| 1 | R1 — G0/1 | Wrong IPv4 address/mask on the Marketing LAN gateway | Set it to 172.16.1.126 255.255.255.192 |
| 2 | R1 | SSH admin account missing or incorrect | Recreate it: username Admin1 secret Admin1pa55 |
| 3 | R1 — VTY lines | VTY lines not restricted to SSH, so SSH access failed | Apply transport input ssh (with login local) |
| 4 | S2 — VLAN 1 | Wrong management IP address, so S2 was unreachable | Set it to 172.16.1.125 255.255.255.192 |
| 5 | IT PC | Wrong IPv4 address and wrong default gateway | IP 172.16.1.1 / 255.255.255.192, gateway 172.16.1.62 |
| 6 | R&D PC | Wrong IPv6 address | Set it to 2001:db8:cafe:2::2/64 (gateway fe80::1) |
No changes needed on: S1, S3, and the Marketing PC.
Addressing scheme (172.16.1.0/24)
| Subnet | Hosts required | Network / CIDR | Usable host range |
|---|---|---|---|
| IT | 50 | 172.16.1.0/26 | 172.16.1.1 – 172.16.1.62 |
| Marketing | 50 | 172.16.1.64/26 | 172.16.1.65 – 172.16.1.126 |
| R&D | 100 | 172.16.1.128/25 | 172.16.1.129 – 172.16.1.254 |
💡 Pro Tip: Struggling with subnetting? Try our VLSM Calculator Online to quickly check your results!
Host fixes (Desktop → IP Configuration)
- IT PC — set IPv4 Address to
172.16.1.1, mask255.255.255.192, Default Gateway to172.16.1.62. (IPv6 is already correct:2001:db8:cafe::2/64, gatewayfe80::1.)
- R&D PC — set IPv6 Address to
2001:db8:cafe:2::2/64, IPv6 gatewayfe80::1. (IPv4 is already correct:172.16.1.129/255.255.255.128, gateway172.16.1.254.)
- Marketing PC — no change needed.
Preconfigured credentials
Enable password: Ciscoenpa55 · Console password: Ciscoconpa55 · SSH account: Admin1 / Admin1pa55
Fix on R1
! ============================================================== !--- 17.8.3 Packet Tracer - Troubleshooting Challenge !--- FIX SCRIPT FOR ROUTER R1 !--- Usage: copy this whole file and paste it into the ROUTER R1 terminal. Log in with the preconfigured credentials (enable: Ciscoenpa55, console: Ciscoconpa55). This is a TROUBLESHOOTING lab: only the corrective commands are applied. ! ============================================================== enable configure terminal ! -------------------------------------------------------------- !--- Fault 1: G0/1 (Marketing LAN) had the wrong IPv4 address/mask, so the Marketing hosts had no working gateway. ! -------------------------------------------------------------- !--- Correct the G0/1 address to the last host of 172.16.1.64/26 (Marketing subnet). interface g0/1 ip address 172.16.1.126 255.255.255.192 no shutdown exit ! -------------------------------------------------------------- !--- Fault 2: the SSH admin account was missing or wrong, so PCs could not log in to R1 over SSH. ! -------------------------------------------------------------- !--- Recreate the local SSH user. Re-issuing this command overwrites any wrong entry. username Admin1 secret Admin1pa55 ! -------------------------------------------------------------- !--- Fault 3: the VTY lines were not restricted to SSH, so SSH access to R1 did not work as required. ! -------------------------------------------------------------- !--- Set the VTY lines to accept SSH and authenticate against the local user database. line vty 0 4 transport input ssh login local exit end ! -------------------------------------------------------------- !--- Save the configuration to NVRAM. (Press Enter when prompted.) ! -------------------------------------------------------------- copy running-config startup-config ! ============================================================== !--- Verification: !--- show ip interface brief -> G0/1 shows 172.16.1.126, up/up !--- show ip ssh -> SSH enabled !--- (from a PC) ssh -l Admin1 172.16.1.62 -> SSH login works ! ==============================================================
Fix on S2
! ============================================================== !--- 17.8.3 Packet Tracer - Troubleshooting Challenge !--- FIX SCRIPT FOR SWITCH S2 !--- Usage: copy this whole file and paste it into the SWITCH S2 terminal. Log in with the preconfigured credentials (enable: Ciscoenpa55, console: Ciscoconpa55). This is a TROUBLESHOOTING lab: only the corrective commands are applied. ! ============================================================== enable configure terminal ! -------------------------------------------------------------- !--- Fault: the VLAN 1 management address was wrong, so S2 was unreachable on the Marketing LAN. ! -------------------------------------------------------------- !--- Correct the VLAN 1 address per the Addressing Table (172.16.1.64/26 subnet). interface vlan 1 ip address 172.16.1.125 255.255.255.192 no shutdown exit end ! -------------------------------------------------------------- !--- Save the configuration to NVRAM. (Press Enter when prompted.) ! -------------------------------------------------------------- copy running-config startup-config ! ============================================================== !--- Verification: !--- show ip interface brief -> VLAN 1 shows 172.16.1.125, up/up !--- ping 172.16.1.126 -> reach the default gateway (R1 G0/1) ! ==============================================================
Verification
- All PCs can ping each other, the switches, R1, and the web server — over both IPv4 and IPv6 (switches are IPv4 only).
- All PCs can reach R1 over SSH using the Admin1 account.
- The web browsers on the IT, Marketing, and R&D PCs can open the web server.
Download Packet Tracer (.pka) file:
Previous Lab
17.8.2 Packet Tracer – Skills Integration Challenge
