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2.1.1. Routers

Think of a router as the post office of networking. When a packet arrives, the router reads the destination IP address (like a ZIP code) and decides which direction to send it. Switches are fast but neighborhood-bound—they only know about local MAC addresses. The moment traffic needs to leave the local network, you need a router. That's why every time you browse the internet, your traffic hits your default gateway first—it's the doorway to everywhere else.

Routers operate at Layer 3 and are the only devices that can forward traffic between different networks. Without routers, your network would be an island—devices could talk locally but never reach the outside world.

What routers actually do:
  • Read destination IPs and consult their routing table to find the best path
  • Separate broadcast domains—broadcasts stop at routers, which is why your DHCP server can't serve the entire building without help
  • Connect to WANs—routers are what link your office to the ISP and the internet
  • Make path decisions—when multiple routes exist, routers choose the best one

What happens without proper routing: Packets destined for remote networks get dropped. Users see "destination unreachable" errors. Nothing outside the local subnet works.

Common Cisco Router Platforms:
  • ISR (Integrated Services Router) 1000/4000 series: Branch office workhorses
  • ASR (Aggregation Services Router): WAN edge, high throughput
  • Catalyst 8000: SD-WAN and cloud connectivity
Router# show ip route
C    192.168.1.0/24 is directly connected, GigabitEthernet0/0
S    10.0.0.0/8 [1/0] via 192.168.1.1
O    172.16.0.0/16 [110/20] via 192.168.1.2