OSI Layers
The 7-layer model, what each layer does, PDUs, protocols, devices, encapsulation, and how to use it to troubleshoot.
Start tutorial →Tutorials
Routing & Switching
MAC tables, VLANs, trunks, STP, routing tables, administrative distance, static routes, OSPF, and inter-VLAN routing.
Start tutorial →ACL & Firewall Rules
Standard and extended ACLs, wildcard masks, rule order, placement, Junos firewall filters, and Linux firewalld.
Start tutorial →How to Use This Site
- Go in order. The OSI model gives you the vocabulary. Routing and switching builds on Layers 2 and 3. ACLs and firewalls build on Layers 3 and 4.
- Lab it. Type the configs yourself in Cisco Packet Tracer, GNS3, EVE-NG, or a Juniper vLab. Reading alone won't stick.
- Check yourself. Each tutorial ends with quick questions. Click a question to reveal the answer.
Tip: When something breaks, start at Layer 1 and work up. Is it plugged in? Is the link up? Is it in the right VLAN? Does it have a route? Is an ACL blocking it?
Quick Reference
| Topic | Key idea |
|---|---|
| Switch | Forwards frames using MAC addresses (Layer 2) |
| Router | Forwards packets using IP addresses (Layer 3) |
| VLAN | Splits one switch into separate broadcast domains |
| Longest prefix match | The most specific route always wins |
| ACL | Processed top-down, first match wins, implicit deny at the end |
| Stateful firewall | Tracks connections and allows return traffic automatically |