Networking concepts, explained
Plain-English answers to the “what is…” questions behind the CCNA.
Read the concept, follow its how-to guide to configure it, then prove it on the free sample lab.
Not sure where a topic fits? The CCNA study hub sequences every concept, guide and lab in blueprint order.
1 Network Fundamentals · 9 explainers
Addressing and the subnet math every later topic is built on. Start here if you are new.
- What Is Subnetting? Subnetting ExplainedSubnetting is the practice of splitting one large IP network into several smaller, self-contained networks called subnets.
- What Is a Subnet Mask?A subnet mask is a 32-bit companion to an IPv4 address that marks where the "network" part of the address ends and the "host" part begins.
- What Is an IP Address? IPv4 vs IPv6An IP address is the logical Layer 3 (Network layer) address that identifies a device on a network and tells the network where to deliver its traffic.
- The OSI Model Explained: All 7 LayersThe OSI (Open Systems Interconnection) model is a seven-layer framework for how data moves across a network, from the physical wire to the application.
- TCP vs UDP: What's the Difference?TCP and UDP are the two main Transport-layer protocols that move your data across a network — but they make opposite trade-offs.
- What Is a MAC Address? MAC vs IP AddressA MAC (Media Access Control) address is a 48-bit hardware address baked into a device's network interface.
- Router vs Switch: What's the Difference?A switch forwards frames by MAC address to connect devices within one network, while a router forwards packets by IP address to connect separate networks.
- Hub vs Switch: What's the Difference?The core difference is the OSI layer they operate at, and everything else follows from that.
- Collision Domain vs Broadcast DomainA collision domain is the set of devices whose frames can collide; a broadcast domain is how far a broadcast reaches. Switches split one, routers the other.
2 Network Access · 4 explainers
Switching: VLANs, trunks, spanning tree and link bundling — how one campus network stays separated and loop-free.
- What Is a VLAN? VLANs Explained for BeginnersA VLAN (Virtual Local Area Network) is a way to slice one physical switch into several separate logical networks.
- What Is Spanning Tree Protocol (STP)?Spanning Tree Protocol (STP) is a Layer 2 protocol that runs on switches to prevent network loops.
- Access Port vs Trunk PortAn access port carries one VLAN and sends frames untagged, while a trunk port carries many VLANs and tags each frame with its VLAN ID using 802.1Q.
- Layer 2 vs Layer 3 SwitchA Layer 2 switch forwards frames by MAC address within VLANs, while a Layer 3 switch adds IP routing to also forward traffic between VLANs and subnets.
3 IP Connectivity · 5 explainers
Routing: how a packet gets from one network to another, by hand first and then dynamically with OSPF.
- How Does Routing Work? The Routing Table ExplainedRouting is how a router moves data between separate networks by reading each packet's destination IP address.
- What Is a Default Gateway?A default gateway is the router interface on your local network that a host sends traffic to whenever the destination is on a different subnet.
- What Is OSPF and How Does It Work?OSPF (Open Shortest Path First) is a link-state interior gateway protocol that lets routers automatically learn the best path to every subnet.
- OSPF vs EIGRP: What's the Difference?OSPF and EIGRP are dynamic routing protocols: OSPF is link-state and multi-vendor, while EIGRP is Cisco's advanced distance-vector protocol.
- Static vs Dynamic Routing: When to Use EachThe core difference is who builds the routing table: with static routing you enter each route by hand, while dynamic routing learns routes automatically.
4 IP Services · 3 explainers
The services a working network runs on — addressing hosts, translating addresses, keeping time, staying reachable.
- What Is DHCP and How Does It Work?DHCP (Dynamic Host Configuration Protocol) automatically hands a device its IP address and other settings the moment it joins a network.
- What Is NAT (Network Address Translation)?NAT (Network Address Translation) is how a router rewrites IP addresses at the network edge so many private devices can share one public IP address.
- HSRP vs VRRP: What's the Difference?HSRP and VRRP let two or more routers share one virtual gateway IP so hosts keep working if the active router fails; HSRP is Cisco, VRRP is an open standard.
5 Security Fundamentals · 2 explainers
Filtering traffic and locking down the devices themselves.
- What Is an Access Control List (ACL)?An Access Control List (ACL) is an ordered list of permit and deny rules a router checks against traffic to decide what it allows and what it drops.
- Standard vs Extended ACLA standard ACL filters on source IP only, while an extended ACL matches source and destination IP, protocol, and port for far more precise rules.
Concept explainers — FAQ
What's the difference between a concept explainer and a how-to guide?
A concept explainer answers “what is it and why does it work that way” — the mental model behind a VLAN, subnet mask, or OSPF adjacency. A how-to guide is the step-by-step Cisco IOS procedure to configure it. Read the concept first to understand the idea, then follow the matching guide to build it.
Are these concepts aligned to the CCNA?
Yes. Every explainer maps to a topic in the Cisco CCNA 200-301 blueprint — addressing and subnetting, switching and VLANs, routing, IP services, and security fundamentals. They're written to build the exact intuition the exam tests.
Where should I start?
Start with the addressing and switching fundamentals — IP addresses, subnet masks, and VLANs — since almost everything else builds on them. For a sequenced path, the study hub orders every concept, guide, and graded lab in blueprint order.
Do I need an account to read these?
No. Every concept explainer is free with no sign-up. When you're ready to prove you can actually configure a topic, the free sample lab grades a real Cisco config with no account or card.
Ready to prove you've got it?
Read the concept, then run the whole graded loop on a real Cisco config — no account, no card.
Keep learning
Study Hub →
A CCNA-aligned roadmap tying every topic, guide, and lab together.
How-to Guides →
Step-by-step walkthroughs for CML, Cisco IOS, and exam topics.
Cheat Sheets →
Printable Cisco IOS command references to keep beside your terminal.
Free Tools →
Subnet & VLSM calculators, a ports reference, and grading drills.