An interface in a routing table specifies the network interface that the router should use to forward packets towards a particular destination.
In more detail, a routing table entry tells a router where to send traffic destined for a specific network. A key component of this entry is the interface. Think of the interface as a physical or virtual port on the router that's connected to the next hop towards that destination. When a router receives a packet, it consults its routing table. If the destination IP address of the packet matches an entry in the table, the router will then forward the packet out of the interface specified in that routing table entry.
Here's a breakdown:
- Destination Network: The network the router needs to reach.
- Next Hop: The address of the next router in the path to the destination network.
- Interface: The specific port or connection on the router that connects to the next hop. It's the physical or logical pathway used to send data.
- Metric: A value that indicates the cost or preference for using this route compared to other possible routes. (While important to routing, it's not the interface.)
Example:
Let's say a router has a routing table entry that looks like this (simplified):
| Destination Network | Next Hop | Interface |
|---|---|---|
| 192.168.2.0/24 | 192.168.1.2 | Ethernet0/0 |
This means:
- If a packet arrives at the router destined for any device on the 192.168.2.0/24 network,
- The router should forward that packet to the next hop router at address 192.168.1.2,
- And the router should send the packet out of its Ethernet0/0 interface.
Without the interface information, the router wouldn't know which of its physical connections to use to forward the traffic, even if it knows the next hop's IP address.
In essence, the interface in a routing table acts as a direction sign, guiding the router on the correct path to send data towards its ultimate destination.
