Routing Table Explained

A Routing Table is the brain of a router (or any Layer 3 device). It is a database stored in the router's RAM that lists all the network destinations the router knows about, and exactly how to reach them. Without a routing table, a router would receive a packet, not know what to do with it, and immediately drop it.

Before reading this, ensure you understand the basic concept of Routing.

What Does a Routing Table Contain?

While the exact output varies between vendors (Cisco, Juniper, Linux, Windows), every routing table entry generally contains the following pieces of information:

• Destination Network: The remote network (e.g., 198.51.100.0).
• Prefix / Subnet Mask: The size of that network (e.g., /24 or 255.255.255.0).
• Next Hop IP: The IP address of the next router in the path to hand the packet to.
• Outgoing Interface: The physical port on the router that the packet should exit (e.g., GigabitEthernet0/1 or eth0).
• Metric / Cost: A numerical value representing how "expensive" or "far" the route is. If the router knows two paths to the same destination, it chooses the one with the lowest metric.
• Route Type: How the router learned this route (e.g., Connected, Static, or Dynamic like OSPF).

Types of Routes

• Directly Connected Routes: These are automatically added to the routing table the moment an administrator assigns an IP address to a router interface and turns the port on. The router inherently knows how to reach networks physically plugged into it.
• Static Routes: Manually configured by an administrator. (See Static vs Dynamic Routing).
• Dynamically Learned Routes: Routes learned automatically from other routers using routing protocols.
• Default Route: The "catch-all" route (0.0.0.0/0). Used when the router doesn't know a specific path to the destination.

The Longest Prefix Match Rule

A router might have multiple routes in its table that match a single destination IP. The universal rule of routing is: The router always chooses the most specific matching route. This is known as the Longest Prefix Match.

Imagine a router has the following three entries in its table:

1. 0.0.0.0/0 (The Default Route, pointing to the Internet via ISP 1)
2. 192.0.2.0/24 (The main remote branch office network, pointing to a VPN)
3. 192.0.2.128/25 (A highly secure subnet inside the branch office, pointing to a specific internal firewall)

If the router receives a packet destined for 192.0.2.150, which route does it take?

• It matches Route 1 (0 bits match, everything matches default).
• It matches Route 2 (24 bits match the network).
• It matches Route 3 (25 bits match the network).

The Winner: Route 3. Because /25 is a longer, more specific prefix than /24 or /0, the router sends the packet to the internal firewall.

Viewing the Local Routing Table

Even your PC has a local routing table to determine if traffic should go to the local Wi-Fi or to a VPN adapter.

Windows

Linux

Common Routing Table Troubleshooting

1. Missing Default Route

If your router can reach internal subnets but cannot reach the internet, check the routing table for 0.0.0.0/0. If it is missing, the router drops all internet traffic because it has no path to the web.

2. Incorrect Next Hop

If a route exists but traffic never arrives, the Next Hop IP might be a dead router, or point to an interface that is down. Use Traceroute to see where the packet is actually being sent.

3. Asymmetric Routing

If PC-A pings PC-B, the packet might take Router 1 to get there. However, PC-B's routing table might dictate that the reply should be sent back via Router 2. If Router 2 has a firewall enabled, it might drop the reply because it never saw the initial connection request. A route must be valid in both directions for two-way communication.

4. Route Exists, Destination Unreachable

A routing table only proves the router thinks it has a valid path. If the destination server is turned off, or a firewall blocks the traffic halfway there, the routing table will still look perfect while the connection fails.

Quick Summary

• A routing table dictates exactly where a router sends every packet it receives.
• Entries contain the destination network, next hop, outgoing interface, and metric.
• The router always prefers the most specific route (Longest Prefix Match).
• Local computers also possess routing tables to manage multiple network adapters.