A firewall is a part of a system or network that blocks unauthorized access but permits outbound communication. They are made to protect systems from security threats such as denial of service attacks(Dos) or data theft. The most critical aspect of an effective firewall is its visibility, which is a key point of value accros different types of solutions including windows firewalls, firewall software, and network firewalls. Ease of use and simplicity of administration are also key points for a firewall because they often function in complex, heterogeneous security enviroments. Firewalls usually consist of specific functions such as intrusion protection(IPS), VPN,data loss prevention, SSL, application control, and web content filtering.