📘 CCNA 200-301 v1.1

1.1.a Routers

🔹 What is a Router?

A router is a device that connects different networks together.

• It looks at the destination IP address inside a packet.
• It decides where to send the packet next.
• Without a router, devices in different networks (or subnets) cannot communicate.

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🔹 What Does a Router Do?

1. Connect networks
   • Example: Connects your school network (LAN) to the internet (WAN).
   • Example: Connects Building A network (192.168.1.0/24) to Building B network (192.168.2.0/24).
2. Path selection
   • The router checks its routing table.
   • If there are many possible paths, it chooses the best one.
   • This can be done with static routes (manually added) or dynamic routing protocols (OSPF, EIGRP, RIP, BGP).
3. Inter-VLAN routing
   • If you create VLANs in a switch, a router (or Layer 3 switch) is needed to let devices in different VLANs talk to each other.
4. Security and filtering
   • Routers can use ACLs (Access Control Lists) to allow or block traffic.
   • This is often the first line of defense before traffic enters your network.
5. NAT (Network Address Translation)
   • Routers can hide private IP addresses behind one public IP.
   • This is how home and school networks access the internet.

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🔹 Router vs Switch

Feature	Router	Switch
Works at	Layer 3 (Network)	Layer 2 (Data Link)
Uses	IP addresses	MAC addresses
Purpose	Connects different networks	Connects devices in the same network
Example	LAN to Internet	Classroom PCs together

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🔹 Key Facts to Remember for CCNA

✅ Router works at OSI Layer 3.
✅ Uses IP addresses for decisions.
✅ Has a routing table.
✅ Can do static routing or dynamic routing.
✅ Provides inter-VLAN routing, NAT, ACLs, and WAN connections.

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🔹 Do different VLANs/Subnets need a router?

Yes ✅.

• Devices in the same VLAN (same subnet) can talk directly through a switch.
• Devices in different VLANs (different subnets) cannot talk directly. They need help from a Layer 3 device.

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🔹 How can different VLANs communicate?

You have two options:

1. Router (traditional way – “Router on a Stick”)
   • Each VLAN is connected to the router through a sub-interface.
   • The router forwards traffic between VLANs.
   • Good for small networks.
2. Layer 3 Switch (modern way – Inter-VLAN routing)
   • A Layer 3 switch can perform routing as well as switching.
   • You create SVIs (Switch Virtual Interfaces) for each VLAN.
   • The switch itself routes between VLANs.
   • Faster and more scalable than using an external router.

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🔹 Key Point for Beginners

• Switch only (Layer 2) → VLANs are isolated.
• Router or Layer 3 Switch → VLANs can communicate.

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✅ For CCNA exam:

• You must know both methods:
  • Router on a Stick (traditional).
  • Inter-VLAN routing on a Layer 3 switch.