Inside the Routing Table

A router job is to forward each packet toward its destination. To do this it consults a routing table, a list of known networks and how to reach them. Every packet that arrives is checked against this table to choose the next step. This lesson opens the routing table and explains the entries a router uses to make forwarding decisions.

Each routing table entry maps a destination network to a next hop and an outgoing interface. The destination is written as a network and prefix, like 10.0.0.0/8. The next hop is the address of the next router on the path, and the interface is the port to send through. Some entries also list a metric and the route source.

The simplest entries are connected routes. When you give a router interface an address on a network, the router automatically knows that network is directly attached. No next hop is needed; the router delivers locally out that interface. Connected routes are always trusted because the router can see the network on its own port.

For networks not directly attached, the entry names a next hop, the IP of the neighboring router that is one step closer to the destination. The router does not need to know the full path, only the next router to hand the packet to. Each router along the way repeats this, forwarding hop by hop until the packet reaches its destination network.

A packet destination might match several entries. The router uses longest prefix match: it chooses the most specific route, the one with the longest matching prefix. For example, a packet to 10.1.1.5 matching both 10.0.0.0/8 and 10.1.1.0/24 follows the /24, since it is more specific. The default route 0.0.0.0/0 is the least specific and matches only when nothing else does.

If two routes to the same network exist within one routing method, the router prefers the one with the lower metric. A metric measures how good a path is, such as hop count or a cost based on bandwidth. Lower means better. The metric helps the router pick the most efficient route among equally specific options.

When routes to the same network come from different sources, the router uses administrative distance (AD) to decide which to trust. AD ranks route sources: connected and static routes are trusted over learned ones. For example, a static route (low AD) is preferred over an OSPF-learned route to the same network. AD picks the source; metric breaks ties within a source.

The entry 0.0.0.0/0 is the default route, the catch-all used when no more specific entry matches. On edge routers it usually points toward the internet provider. Without a default route, a router drops packets to unknown networks. With one, anything unfamiliar is simply forwarded out toward the wider internet, which is exactly what small networks want.

You view a routing table with a command like show ip route on a router or the route print or ip route tools on hosts. Letters often flag each entry source: C for connected, S for static, O for OSPF, and so on. Learning to read this output is a vital skill, it tells you exactly how a router will forward any given packet.

Each table entry comes from a source. Connected routes are networks on the router own interfaces. Static routes are entered by an administrator. Dynamic routes (like those from OSPF) are learned automatically from a routing protocol. The router shows the source with a code letter and uses it, via administrative distance, to decide which to trust when sources overlap. Knowing the source explains why a particular route is in the table.

The routing table is the brain of forwarding. Each entry pairs a destination network with a next hop and interface. The router applies longest prefix match to pick the most specific route, uses administrative distance to choose among sources, and metric to break ties. Understanding these rules lets you predict and troubleshoot exactly how traffic flows, a core Network+ skill.

Test your routing table knowledge.

A routing table maps each destination network to a next hop and interface. Connected routes are directly attached networks. Routers pick paths by longest prefix match (most specific wins), use administrative distance to choose among route sources, and metric to break ties within a source. The default route 0.0.0.0/0 catches anything unmatched. Reading the table shows exactly how a router forwards each packet.