#TechExchange: What are the contents of a Routing Table?

Routing is a technique used to forward data packets at Internet level on electronic communication systems. This is one of the core concepts in the integration of modern day networks whereby every Internet user is able to reach global servers to access information from remote sites.

By Cisco Eng. Shingie Lev Muringi 

So routing tables contain the vital information, including the shortest possible path on which a data packet will hop from the sender node to the final destination node. What other vital information is available in the routing table to help networks when forwarding data??        

On a Windows host, the route print or netstat -r command can be used to display the host routing table. Both commands generate the same output. The output may seem overwhelming at first, but is fairly simple to understand.

Routing Table1
A proper routing table from a windows host

Entering the netstat -r command or the equivalent route print command, displays three sections related to the current TCP/IP network connections:

  • Interface List – Lists the Media Access Control (MAC) address and assigned interface number of every network-capable interface on the host including Ethernet, Wi-Fi, and Bluetooth adapters.
  • IPv4 Route Table – Lists all known IPv4 routes, including direct connections, local network, and local default routes.
  • IPv6 Route Table – Lists all known IPv6 routes, including direct connections, local network, and local default routes.

Note: Command output varies, depending on how the host is configured and the interface types it has.

The figure displays the IPv4 Route Table section of the output. Notice the output is divided into five columns which identify:

  • Network Destination – Lists the reachable networks.
  • Netmask – Lists a subnet mask that informs the host how to determine the network and the host portions of the IP address.
  • Gateway – Lists the address used by the local computer to get to a remote network destination. If a destination is directly reachable, it will show as “on-link” in this column.
  • Interface – Lists the address of the physical interface used to send the packet to the gateway that is used to reach the network destination.
  • Metric – Lists the cost of each route and is used to determine the best route to a destination.

To help simplify the output, the destination networks can be grouped into five sections as identified by the highlighted areas in the figure:

0.0.0.0

The local default route; that is, all packets with destinations that do not match other specified addresses in the routing table are forwarded to the gateway. Therefore, all non-matching destination routes are sent to the gateway with IP address 192.168.10.1 (R1) exiting from the interface with IP address 192.168.10.10. Note that the final destination address specified in the packet does not change; rather, the host simply knows to forward the packet to the gateway for further processing.

127.0.0.0 – 127.255.255.255

The battle for relevance continues…follow Shingie Levison Muringi our Technology Research Specialist and Sub Editor on Twitter @ShingieMuringi1, Email shingie@technomag.co.zw or direct Cell: 0775 380 652 for all the latest trending technological issues in and outside Zimbabwe.

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