When Company A and Company B merged, the network security administrator for Company A was tasked with joining the two networks. Which of the following should be done FIRST?

Implementing a unified IPv6 addressing scheme is an infrastructure reconfiguration task that should only happen after the security posture of both networks has been assessed and any risks mitigated.

Penetration testing is an active, adversarial assessment technique that is typically scoped and executed after a vulnerability assessment has already mapped the attack surface to prioritize.

A vulnerability assessment is a passive, non-destructive evaluation that catalogs known misconfigurations, unpatched software, and policy gaps on Company B's network before any integration takes place. Running it first prevents the administrator from inadvertently bridging Company A's network to an already-compromised or weakly secured environment. The assessment results also serve as the security baseline required to plan all subsequent integration and remediation steps safely.

A peer code review evaluates the correctness and security of application source code, which is unrelated to the immediate task of safely integrating two network infrastructures.