What is a firewall? At which layer of the OSI model it works?

 The firewalls work at Layer 3 (Network Layer), Layer 4 (Transport Layer), and Layer 7 (Application Layer).

• A firewall is a network security device that acts as a protection layer between incoming and outgoing traffic. 
  
  
• It allows the traffic to enter and leave the network in coordination with pre-defined rules.
  
  
• It is a device that provides security to the users as it is placed between a trusted and an untrusted network. 
  
  
• Its purpose is to provide security to users, servers, and internal networks.