CCNA 2: Routing Protocol and Concepts: Intro to Routing and Packet Forwarding: The Routing Table

This is a continuation of “Inside the Router”.

What is a routing table? You wouldn’t have a clue if you don’t know what a router is. As a review, routers forward data packets to their destination networks. Before they do, routers have to know where to send the data packets, where these destination networks are. How? They keep a table of routing information, the routing table. It is a data structure in the form of a table-like object stored in a router. It contains information about directly connected and remote networks.

image from http://rekrowten.files.wordpress.com/

image from http://rekrowten.files.wordpress.com/

Above is an example of a Cisco Router’s routing table. It displays the network address and the subnet mask, indicating the destination network and how it is connected to the current router. Destination networks can either be directly (physically) connected to the router or not (remote networks – networks that a few or more routers away from the current one).

There are thousands of routers that make up the internet. You can imagine how many networks exist in this world.

How do routers know all these networks? They initially don’t. There are three ways a router learns about the existing networks (both directly connected and remote) – which build up the routing table.

  1. Directly Connected Routes – These are networks physically connected or directly attached to the router. When you create a LAN and connect it to a FastEthernet port on a router, you are populating that router’s routing table by adding one network that is directly connected to it.
  2. Static Routes – These are routes to networks that are manually assigned by a network administrator. Being a static route means, the routing info doesn’t change unless manually done so by a network admin. Additionally, and the best way to further emphasize this, from the CCNA material - A train uses the same railroad tracks every time for a specified route. This path is similar to a static route because the path to the destination is always the same.
  3. Dynamic Routes – Assigning IP addresses to computers on a network can be done either statically or dynamically (DHCP). This can be compared to dynamic routing. There are dynamic routing protocols that make it possible for routers to dynamically or automatically learn routes or paths to remote networks (those that are not directly attached to it) without needing to be manually assigned and updated by a network administrator.

A router can use one or a combination of all the above mentioned ways to learn routes to networks and populate its routing table to be used when deciding where and how to forward a packet to its destination network.

On the next chapters, we will learn about several dynamic routing protocols for IP that are being used in today’s networks.

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