Application Gateways / firewallThe application level firewall is installed on a dedicated computer; also called as a proxy server. These servers can store the recently accessed pages in their cache and called as cache servers. As proxy server is placed in unsecured area of the network (for example DMZ), it is exposed to higher levels of risk from unreliable networks. Additional filtering routers can be implemented behind proxy server, further protecting internal systems. The disadvantage is they are characteristically restricted to a single application, as they work at application layer. Figure given below illustrates the different types of Firewalls which is compared to OSI model
Circuit GatewaysThe circuit gateway firewall operates at transport layer. Filtering firewalls, do not normally look at data traffic flowing between two networks, but it prevent direct connections between one network and the other. This is can be accomplished by creating tunnels connecting specific processes or systems on each side of firewall, and allows authorized traffic in the tunnels. MAC Layer FirewallsMAC layer firewalls which is designed to operate at media access control layer of OSI network model. This gives the ability to consider specific host computer’s identity in the filtering decisions of it. The MAC addresses of specific host computers are linked to access control list (ACL) entries that identify specific types of packets which can be sent to each host; all other traffic is blocked. Hybrid FirewallsHybrid Firewalls combine elements of other types of firewalls; that is, elements of packet filtering and proxy services, or of packet filtering and circuit gateways. On the other hand, it may consist of 2 separate firewall devices; each is a separate firewall system, but is connected to work in tandem. Without replacing the existing firewalls completely, an organization can make a security improvement, from this approach.