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Routing

How routers move packets between networks: reading the routing table, choosing between routes, building static and default routes by hand, and letting OSPF and BGP do the work.

Intermediate · 13 lessons incl. lab and test · Before this: IP Addressing, Subnetting

Routing is the process a router uses to forward a packet toward a destination on another network. It compares the destination IP address with the routes in its routing table, which can be connected, static or learned from a dynamic routing protocol, and sends the packet to the next hop of the best match.

In simple terms: A router looks at where each packet is going, checks its list of known networks and passes the packet one step closer, like a sorting office forwarding a letter to the next town.

Routing is how a packet gets from one network to another, possibly through many routers. Every router on the way makes its own decision: for this destination, where does the packet go next? This course explains that decision and every way a router can learn its routes, using the same three-router network in every lesson.

192.168.1.0/24203.0.113.0/3010.0.12.0/3010.0.23.0/30192.168.2.0/24192.168.3.0/24PC1192.168.1.10ISP203.0.113.1R1R2R3PC2192.168.2.10Server192.168.3.10
  1. 1. Between LANs: PC1's packet crosses R1, R2 and R3. Each router picks the next hop from its own routing table.
  2. 2. To the internet: anything the routers don't know follows default routes toward the ISP.

What you'll learn

How routers decide

The forwarding decision, the routing table, longest prefix match, administrative distance and metric.

Static routing

Static, default and floating static routes on Cisco routers, then a lab that ties them together.

Dynamic routing

Why routing protocols exist, how OSPF finds the lowest-cost path, and where BGP fits on the internet.

Troubleshooting

A step-by-step method with ping, traceroute and show ip route, including one-way traffic.

Skills you'll gain

  • Routing tables
  • Longest prefix match
  • Administrative distance
  • Static routes
  • Default routes
  • Floating static routes
  • OSPF basics
  • BGP basics
  • Troubleshooting

Who is this for?

CCNA students and anyone who wants to understand how traffic moves between networks. You should be comfortable with IP addresses and subnet masks first. If /24 and /30 mean something to you, you're ready.

Start the first lesson →

FAQ

What's the difference between a default gateway and a next hop?
A default gateway is a setting on an end device: the router it sends traffic to when the destination isn't on its own subnet. A next hop is the general routing idea: the very next device a packet is handed to on its way. Every route in a router's table has a next hop; a PC's default gateway is just the first one.
Why not just use static routes everywhere? They sound simpler.
Static routes are fine for small, stable networks: a few routers and links that rarely change. They stop scaling once there are many routers or the topology changes often, because every change means editing every affected router by hand, and they can't react to failures further away. Dynamic routing protocols solve that by letting routers share routes and adjust on their own.
Do I need to understand OSPF and BGP in depth?
For the CCNA you need to configure and verify single-area OSPF, which a separate OSPF course covers in full. BGP is mostly for ISPs and large organisations; knowing what it is for and how it differs from OSPF (OSPF inside one organisation, BGP between organisations) is enough for most jobs.