Simple definition
Unicast routing forwards IP packets to a single destination by choosing next hops from a routing table.
Purpose and use
Unicast routing forwards a packet toward one IP destination using a routing table and next hop.
How does it work?
Lookup generally selects the longest matching installed prefix, then uses that route's eligible forwarding paths.
Example and verification
For an unreachable destination, check the selected route, neighbor or next-hop resolution and return path.
Points to watch
A route does not prove that a firewall permits traffic or a remote application is listening. A default gateway cannot override every more-specific route.
In short
Diagnosis should separate path selection, next-hop reachability, filtering and application response.