Firewall layer
Network stack
Firewalls trap rogue use of your network. In a world where one person's idea of fun is to break into another person's private domain, firewalls have become an essential component in limiting the damage that can be done.
Firewalls can be simply understood and implemented with iptables—a service running on your back-end server—or with dedicated hardware devices custom built to filter network packets. Both tools operate at the port level, allowing or disallowing use of services into, or out of, your network, on a port-by-port basis.