Firewall in computer networks is an essential component of network security. It acts as a barrier between an internal network and external networks, such as the internet, and its main purpose is to protect against unauthorized access and control incoming and outgoing network traffic.

Firewalls operate based on predefined rules and policies, which determine how traffic is allowed or blocked. They examine packets of data entering or leaving the network and compare them against these rules. If a packet meets the criteria defined in the firewall rules, it is allowed to pass through, otherwise, it is blocked.

Here are the main reasons why firewalls are used in computer networks:

1. Protection against unauthorized access: Firewalls are designed to prevent unauthorized users or malicious entities from gaining access to a network. By filtering incoming network traffic, firewalls can block unauthorized access attempts, such as hacking attempts or unauthorized remote connections.

2. Control of incoming and outgoing network traffic: Firewalls provide control over the flow of network traffic. They can be configured to allow or block specific types of traffic based on various criteria, such as source IP address, destination IP address, port numbers, or protocols. This helps in preventing the spread of malware, blocking spam, and securing sensitive data.

3. Network segmentation: Firewalls can be used to divide a network into smaller segments, known as subnets or virtual LANs (VLANs). This allows for better control and security within the network by limiting the communication between different segments. Each segment can have its own set of firewall rules and policies.

4. Protection against network-based attacks: Firewalls provide protection against various network-based attacks, such as Denial of Service (DoS) attacks, where an attacker tries to overwhelm a network with excessive traffic, or Distributed Denial of Service (DDoS) attacks, where multiple compromised systems are used to target a network.

5. Logging and monitoring: Firewalls often have logging and monitoring capabilities that enable network administrators to track and analyze network traffic. This helps in identifying potential security breaches, detecting suspicious activities, and investigating incidents.

In conclusion, the purpose of a firewall in computer networks is to safeguard the network from unauthorized access, control incoming and outgoing network traffic, protect against network-based attacks, and provide network segmentation. By implementing a firewall, organizations can enhance their network security and protect sensitive data from unauthorized access.