The OSI model, which stands for the Open Systems Interconnection model, was made in the early 1980s by the International Standards Organization. The OSI model promotes interoperability of network devices manufactured by different companies. There are seven layers to an OSI model. They are Application, Presentation, Session, Transport, Network, Data Link and Physical.
The seventh layer, Application, supports application and end user process. Primary functions of the application layer include "file transfer, resource allocation, and the identification and verification of computer availability".
The sixth layer is the Presentation layer. It is used to translate data so that it can be read by the receiver. It encrypts data so that it can be sent back and forth on a network. When users view .jpg images or listen to .flac music files, they are interacting with the presentation layer of the OSI model.
The fifth layer is the Session layer. It controls the communication between computers. It also establishes, manages and terminates the connections between the local and remote application. Session layer protocols also format the data sent over the communications medium so the destination computer can use it.
The fourth layer is the Transport layer. It manages data transfer and CRC checks. TCP is a transport layer protocol that ensures data is sequenced properly and is received as it was sent before a destination computer attempts to use it.
The third layer is the Network layer. It addresses data and manages delivery between networks. An example of the network layer is Internet Protocol.
The second layer is the Data Link layer. It provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer.
The first layer is the physical layer. It is referred to as the network medium. It contains the cables, plugs, etc. This layer also conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level.
Packet-filtering routers operate at the third layer of the OSI model, the network layer. These routers protect the networks they are connected to by allowing packets to pass through only if those packets are allowed according to a set of preconfigured rules. For instance, UDP packets are only allowed to pass to server A, but a UDP packet is addressed to server B; the packet will be discarded to protect server B from disallowed traffic.
Firewalls are also known as application-level gateways, and as such operate on the seventh layer of the OSI model. In order to be effective, firewalls must sit between the internal network and the external network. Firewalls rely on a special set of rules called proxy services that must be installed. A proxy service is unique to an application, and therefore creates additional overhead in the form of time required to set up the firewall. If an application is asking to cross the firewall and its proxy service is not installed, configured improperly, or disallowed, the request will be denied. Though the increased overhead is substantial, the network administrator has more control over what packets pass through the firewall in either direction.
The OSI model is essential for the correct interoperability of dissimilar computer networks. Each layer of the model performs a specific function necessary for the proper routing of data from its originator to its destination. External entities pose a threat to network security, making additional hardware or software necessary to protect the CIA triad of information security. Packet-filtering routers, which operate on layer three of the OSI model, and application-level gateways (firewalls) serve to reject data packets that a network administrator deems as potentially dangerous, thereby bolstering the security of a network.