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 Security groups, Network ACL and Firewall Stateful and Stateless firewall - when connection initiated from client firewall checks the rules written and based on the defined rules allows/denies the traffic. Client initiates connection from any port (except the reserved by the OS) but destination port is known and static. So stateful firewall which as name implies keeps the state of connection so that even though there is no outbound rule for the outgoing connection, firewall will automatically allow it. Contrary to this approach in stateless firewall case you should explicitly define rule for inbound and outbound connections.   Network Access Control List (Network ACL) -  is stateless firewall allows you to define rules in subnet level. Also it processes rules in order. Security group - on the other hand stateful and also applied to individual instance or instance group . It evaluates all the rules.  Let's learn how to manage Network ACL's : Search VPC in the s...

OSI model consists of 7 layer : 1. Physical layer - Raw bits transfered over cable via electrical signals. 2. Data link - Switches direct these electrical signals. They use MAC address to identify the next destination.  3. Network layer - Routers receive frames from Data link layer and directs them to the destination based on the address defined in the frame. They use IP (Internet protocol) to identify the next destination. ICMP, ARP are example protocols work on this layer. 4. Transport layer - This layer manages delivery and error checking. TCP,UDP are the examples of protocols work on this layer. 5. Session layer - This layer controls communication between two computers. Netbios,PPTP is the one of the example works on this layer. 6. Presentation layer - This layer is responsible for encryption/decryption of data. SSL, TLS are examples. 7. Application layer - The final layer where user interacts. Example protocols : HTTP,SMTP, FTP etc. So we can say that switches connect computer...