A policy boundary is an enforced rule set that defines which internal connections are allowed. In microsegmentation, it acts like a gate between workloads, services, or network zones, permitting only the traffic that matches policy and blocking everything else by default. This is more precise than a broad perimeter firewall because the boundary can follow applications, identities, or specific ports and protocols.
Policy boundaries matter because they limit lateral movement after an attacker gains access. If one host is compromised, a tight boundary can stop the intruder from reaching databases, admin tools, or other servers. In defense, teams build and maintain these rules from real traffic patterns, asset inventories, and change management. If the boundary is too loose, it does little; if it is too strict, it can break legitimate services. Good policy boundaries are accurate, auditable, and updated as systems change.