Reachability verification is the process of checking whether traffic can actually move from a source to a destination under the network’s current configurations. It goes beyond reading firewall policy on paper and tests the live effect of routing, ACLs, security groups, NAT, segmentation, and other controls along the path.
It matters in cyber security because a rule that appears to allow access may still be blocked by another device, while an old exception may continue to permit traffic that should no longer flow. Defenders use reachability verification to validate change requests, find unintended exposure, and confirm that critical paths are still open after policy updates. Attackers may benefit when hidden paths or stale rules create unexpected access between systems. In practice, the result helps teams prove what the network really permits, not just what the documentation says it should permit.