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OSI, topology and labs →

// Unofficial educational resource — not affiliated with or endorsed by Cisco Systems, Inc. Cisco®, IOS®, Packet Tracer® and CCNA® are registered trademarks of Cisco Systems, Inc.

// learning path

From concept to execution

Follow the path in order, or jump to any stage.

OSI Model
Concept
→
Network Map
Context
→
Commands
Reference
→
Terminal Lab
Execution
→
Troubleshooting
Diagnosis

// all modules

Everything in one place

learn — concept

OSI Model

7 layers with protocols, common faults and CLI commands. The hub connecting theory to every other tool.
practice — execution

Terminal Lab

Interactive Cisco IOS simulator. 8 fault scenarios — run real commands, see real output.
fix — diagnosis

Troubleshooting

OSI-based playbooks for the 5 most common Packet Tracer faults, with step-by-step commands.
learn — step-by-step

PT Guides

Routing, VLANs, DHCP, NAT/PAT, ACLs, IPv6, SSH — every step with CLI output.
fix — reference

Command Library

Searchable Cisco IOS command reference organized by category, including IPv6 and NAT.
explore — topology

Network Map

Visual topology linked to Terminal Lab scenarios. See the network before you type.
practice — quiz

Quiz

Multiple-choice questions on subnetting, VLANs, routing, NAT and security.
practice — subnetting

Subnetting Trainer

Calculate network addresses, broadcasts and host ranges. Includes binary visualizer.
explore — explainer

Config Explainer

Paste any IOS config and get a plain-English breakdown of every line.

// new here?

Never used Cisco before?

Start here.

Cisco IOS is the operating system on Cisco routers and switches. You control it through a text CLI — type commands, configure interfaces, set up routing.

Packet Tracer is Cisco's free simulation tool. Build virtual networks without physical hardware — the standard tool for CCNA study.

Start with PT Guides for step-by-step walkthroughs, or open the Terminal Lab to run live IOS commands right now.

// related tool

Config Generator

Build complete Cisco configurations visually — routers, switches, VLANs, DHCP, OSPF. Paste the output straight into Packet Tracer.

Open Config Generator ↗

// 01 — commands

Command Library

Searchable reference of common Cisco IOS commands, organized by category.

// see also:
>_

CommandDescriptionExampleCategory

// 02 — subnetting

Subnetting Trainer

Practice calculating subnet values. You get a random IP and prefix — figure out the rest.

How subnetting works: An IP address has two parts — the network portion (set by the prefix length) and the host portion. The network address (all host bits = 0) identifies the subnet itself and cannot be assigned to a device. The broadcast address (all host bits = 1) is used to reach all devices in the subnet simultaneously and cannot be assigned either. That is why usable hosts = 2ⁿ − 2, where n = number of host bits.

Given IP address

—

Correct: 0 Attempts: 0 Streak: 0 Best streak: 0

// ipv6 trainer

IPv6 Subnetting Trainer

IPv6 uses 128-bit addresses in hex. A /64 is the standard LAN prefix — the first 4 groups are the network, the last 4 are the host ID.

How IPv6 addressing works: An IPv6 address has 8 groups of 4 hex digits. With a /64 prefix, the first 4 groups identify the network — the last 4 groups are the host ID. The network address zeros out the host groups, written as 2001:db8:a:1:: (double-colon = consecutive zero groups). There is no broadcast in IPv6 — multicast is used instead.

Given IPv6 address

—

PrefixUse caseHost bitsIPv4 equivalent
/32ISP allocation to a customer site96 bits~/8
/48Enterprise site prefix80 bits~/16
/56Home network (ISP to customer)72 bits—
/64Standard LAN segment64 bits~/24
/126Point-to-point WAN link (2 hosts)2 bits/30
/128Single host / loopback0 bits/32

// vlsm

VLSM — Variable Length Subnet Masking

Instead of splitting a block into equal subnets, VLSM lets you use different prefix lengths for different needs — saving address space.

Example: You have 192.168.1.0/24 and need to allocate:

Step 1 — sort by size, largest first: LAN-A (60 hosts), LAN-B (28 hosts), LAN-C (12 hosts), WAN link (2 hosts)

Step 2 — assign smallest fitting prefix to each:
  LAN-A: needs 62 usable → /26 (64 total, 62 usable) → 192.168.1.0/26
  LAN-B: needs 30 usable → /27 (32 total, 30 usable) → 192.168.1.64/27
  LAN-C: needs 14 usable → /28 (16 total, 14 usable) → 192.168.1.96/28
  WAN: needs 2 usable → /30 (4 total, 2 usable) → 192.168.1.112/30

Key rule: each new subnet starts right after the previous block ends. Never overlap.
SubnetNetworkRangeBroadcastUsable
LAN-A /26192.168.1.0.1 – .62192.168.1.6362
LAN-B /27192.168.1.64.65 – .94192.168.1.9530
LAN-C /28192.168.1.96.97 – .110192.168.1.11114
WAN /30192.168.1.112.113 – .114192.168.1.1152

// reference

CIDR Quick Reference

Common prefix lengths and their subnet masks.

PrefixSubnet MaskUsable HostsUse Case
/30255.255.255.2522WAN point-to-point links
/29255.255.255.2486Small WAN / management
/28255.255.255.24014Small office segment
/27255.255.255.22430Small LAN
/26255.255.255.19262Medium LAN
/25255.255.255.128126Half a /24
/24255.255.255.0254Standard LAN — most common
/23255.255.254.0510Larger LAN
/22255.255.252.01022Campus network segment
/16255.255.0.065534Large enterprise / ISP block

// 03 — explainer

Config Explainer

Paste any Cisco IOS configuration and get a plain-English explanation of every line.

// 04 — guides

Packet Tracer Guides

Step-by-step configuration walkthroughs for common Cisco Packet Tracer scenarios.

// 06 — troubleshooting

Troubleshooting Methodology

Systematic OSI-based approach to diagnosing and fixing network faults in Cisco environments.

Bottom-up approach: Always start at Layer 1 (Physical) and work your way up. A routing problem at Layer 3 is pointless to debug if the cable at Layer 1 is unplugged. Confirm each layer before moving to the next. View the OSI model →
LayerWhat to checkKey commandsCommon fault
L1 Physical → Cable connected, port not disabled, correct media show interfaces Status: down/down — cable fault or shutdown
L2 Data Link → Correct VLAN, trunk config, no duplex mismatch show vlan brief
show interfaces trunk
show mac address-table		 Status: up/down — encapsulation or VLAN mismatch
L3 Network → IP address, subnet mask, default gateway, routing table show ip interface brief
show ip route
ping / traceroute		 Wrong mask, missing route, wrong gateway
L4 Transport → Correct port open, ACL not blocking, NAT translations show ip nat translations
show access-lists
telnet <ip> <port>		 ACL denying traffic, NAT not translating

// step-by-step playbooks

Common Scenarios

Click a scenario to expand the diagnostic steps.

// 05 — quiz

Test Your Knowledge

Multiple-choice questions on Cisco networking. Up to 20 questions, randomized order.

🏆
Achievement Unlocked

Profile

> // choose avatar

> // achievements

Badges earned through your learning journey.

// ★ terminal lab

Terminal Lab

Interactive Cisco IOS shell. Pick a scenario, enter privileged mode with enable, and troubleshoot.

💻
PC
192.168.1.10
🔀
Router
NetCLI
☁️
ISP
8.8.8.8
NetCLI>

Try: enable → show ip interface brief → ping 8.8.8.8 · Tab completes · ↑/↓ history · clear clears screen

// ▣ network map

Network Map

Live view of the active terminal scenario — devices, interfaces and link health.

Active scenario: —

// links

// ▤ osi model

The OSI Model

Seven layers — from copper to code. Click a layer to expand examples, common faults and the Cisco CLI commands that diagnose each one.

Why it matters: The OSI model is the mental map every network engineer uses to isolate faults. When something breaks, you don't guess — you walk the stack. Start at Layer 1 (is the cable in?) and move upward only when the layer below is confirmed healthy. Each layer adds its own header, its own concerns, and its own show commands.
Mnemonic (bottom → top): Please Do Not Throw Sausage Pizza Away — Physical · Data Link · Network · Transport · Session · Presentation · Application.

// legal

Legal & Privacy

Last updated: April 2026 · Publisher: SCHJOLDR

GDPR · Privacy Policy

> // Privacy Policy

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Data processors

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Retention

Data is stored as long as the account is active. You can delete your account and all associated data directly in Profile → Delete account.

Your rights (GDPR Ch. III)

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For privacy enquiries: support@schjoldr.io

Age requirement

This service is not directed at children under 13.

Terms of Use

> // Terms of Use

The service

cisco:learn is a free learning platform for Cisco IOS and networking technology. The service is provided free of charge and without any warranty.

Disclaimer

  • The service is provided "as is" without warranties of availability, accuracy or fitness for any purpose.
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  • SCHJOLDR is not liable for any damage resulting from use of or inability to access the service.

Governing law

These terms are governed by Norwegian law.

Copyright

> // Copyright

© 2026 SCHJOLDR. All rights reserved.

Cisco®, IOS®, Packet Tracer® and CCNA® are registered trademarks of Cisco Systems, Inc. This app is an independent educational resource and is not affiliated with, endorsed by, or sponsored by Cisco Systems, Inc.

// learn — protocol numbers

IP Protocol Numbers

The IP header's Protocol field (8 bits) identifies what's inside the packet. These are the numbers that matter for Cisco networking and troubleshooting.

What is a protocol number? Every IP packet has a Protocol field in its header (byte 10). This tells the receiving device what comes next — TCP (6), UDP (17), OSPF (89), etc. ACLs that match ip traffic see all of these. To match specifically, use the protocol number or keyword (permit tcp, permit ospf).

Not the same as port numbers. Protocol numbers identify the transport layer or encapsulation. Port numbers live inside TCP/UDP segments and identify the application. See for ports.

// learn — port reference

Port Reference

Well-known TCP and UDP ports relevant to Cisco networking, Packet Tracer labs and ACL configuration. Organised by function, not by number.

0 – 1023 · Well-known (IANA assigned) 1024 – 49151 · Registered (vendor/app use) 49152 – 65535 · Dynamic / Ephemeral (client source ports)
ACL tip: Extended ACLs match both source and destination port. To permit inbound SSH: permit tcp any host 10.0.0.1 eq 22. To permit DNS responses back: permit udp any eq 53 any. For return TCP traffic use established or a stateful firewall. See for the IP-level protocol field.