Firewall
A firewall is a software or hardware-based network security system that acts as a barrier between an internal network and the external network (usually the internet). It monitors and controls incoming and outgoing network traffic based on predetermined security rules.

Preventing External Access
One of the primary functions of a firewall is to prevent unauthorized external access to a system. It does this by examining each packet of data that tries to enter or leave the network and determining whether it should be allowed or blocked based on the defined rules.

Packet Filtering
Firewalls use packet filtering to examine the header information of each data packet and decide whether to allow or deny it. The header contains details such as the source and destination IP addresses, port numbers, and protocol type. By analyzing this information, the firewall can enforce rules that restrict access to specific ports or IP addresses, effectively preventing external access to the system.

Stateful Inspection
Firewalls can also employ stateful inspection, which goes beyond simple packet filtering. It keeps track of the state of network connections and only allows packets that are part of an established connection or meet specific criteria. This helps prevent unauthorized access by ensuring that only legitimate packets are allowed through.

Application-Level Gateways
Some firewalls also include application-level gateways (proxies) that provide an additional layer of security. These gateways act as intermediaries between the internal network and the external network, inspecting the content of the data packets at the application layer. This allows the firewall to detect and block potentially malicious or unauthorized traffic, preventing external access to the system.

Conclusion
In summary, a firewall is the software that primarily prevents external access to a system. It achieves this by using packet filtering, stateful inspection, and application-level gateways to control and monitor network traffic, ensuring that only authorized and legitimate connections are allowed. By implementing a firewall, organizations can enhance their network security and protect their systems from unauthorized access and potential threats.