Cisco Certified Network Associate (CCNA) 200-301 · Free study guide

Objective 3.1 — Interpret IPv4 routing tables

A routing table records destinations the router currently considers usable. Read one entry as a complete statement: route source, destination prefix, preference, metric, next hop or exit interface, and sometimes age. One isolated number rarely answers the forwarding question.

Route source codes

Common codes distinguish connected, local, static, and dynamically learned routes. A connected route appears for the subnet of an operational addressed interface. A local host route represents the interface's own IPv4 address. A static route comes from configured intent. OSPF routes are learned through that protocol. Codes tell origin, not automatically which matching route wins.

Prefix and mask

The network and prefix length define the set of matching destination addresses. A /24 and /25 that start with similar text are different routes. Expand the mask mentally or calculate the boundary before selecting an entry. The route table can contain overlapping prefixes deliberately.

Next hop and exit interface

A next-hop address names the adjacent router that should receive the packet. An exit interface identifies where to send it. Some entries include both. A next hop on a multiaccess network usually requires recursive resolution and neighbor resolution. The router must know how to reach that next hop through a connected or otherwise resolvable route.

Administrative distance and metric

Bracketed fields commonly show administrative distance and metric. Distance ranks different route sources for the same destination prefix. Metric ranks candidate paths within a routing protocol. A low OSPF metric cannot make a /16 defeat a matching static /24; specificity is considered in forwarding.

Gateway of last resort

The gateway of last resort reflects the selected IPv4 default route. It is used only when no more-specific destination route matches. A default configured in text may not become active if its next hop cannot resolve. Verify both the default entry and the gateway-of-last-resort indication.

Worked output

For destination 10.8.4.50, a table contains connected 10.8.4.0/24, OSPF 10.8.0.0/16 [110/20], and static default [1/0]. The connected /24 is the most-specific match. The lower distance of the default is irrelevant because it is less specific. For 203.0.113.7, if no specific route exists, the default can apply.

Verification workflow

Find all prefixes that contain the destination and select the longest. Read the chosen source, distance, metric, and next hop. Resolve the next hop and verify the exit interface is operational. Compare the control-plane route with a targeted forwarding or trace test where policy permits.

Common traps

• Choosing the lowest distance before longest prefix.
• Treating route age as metric.
• Confusing a local host route with a remote host route.
• Assuming a configured default is active.
• Ignoring recursive next-hop reachability.

Readiness checklist

• I can explain every field in a route entry.
• I distinguish source, distance, and metric.
• I can identify the active default.
• I can resolve a selected next hop to an exit path.