A device essential for blocking unauthorized access in a network is a firewall. Firewalls act as a barrier between trusted internal networks and untrusted external sources by monitoring incoming and outgoing traffic based on predetermined security rules. By filtering data packets, firewalls prevent cyber threats such as unauthorized access, malware attacks, and data breaches. They can be implemented as hardware devices, software applications, or a combination of both, and are crucial in establishing a secure network environment for organizations of all sizes.