The seventh layer is the application layer and it helps one computer talk to another computer through the network. The application layer compiles the data that needs to be sent with an application and then sends it to the presentation layer.
The sixth layer is the presentation layer which helps illustrate the data. The presentation layer adds a header to the data that holds the requests and information if applicable. When sending data it encrypts then compresses data to send to the session layer. When receiving data it decrypts and unpacks the information for the application layer.
The fifth layer is the session layer and it is in charge of administrative and security functions. This layer is also responsible for the connection that tales place between two network devices.
The transport layer is the fourth and it makes sure the data is transferred without errors. The transport layer takes the data packet and breaks it into smaller pieces. When this layer receives data it makes sure the files are in order then turns the several pieces back into one piece.
The third layer is the network layer and it makes sure the data is in order when it moves from one node to another on a network. If the data is out of order when it receives it the data link layer will put it back in order. This layer is also responsible for routing decisions, connecting networks, stopping the connections and keeping the connections going.
The second layer is the data link layer and it is in charge of moving data over the conduit. When sending data this layer will make sure there is a link available for transmission. Then it will convert the “data into it streams for delivery to the lowest layer.” (Merkow & Breithaupt, 2006, p. 262).The data link layer will receive data from the NIC and it removes the first header.
The first layer is the physical layer and it takes care of the physical signal. This layer includes everything that is physical in a network. The physical layer is responsible for transporting the data from one Network device to another.
Packet filtering Routers reside on the three lower layers of the OSI model. The physical layer is the layer that allows more than one device to communicate with each other. The data link layer makes sure the transmission of data has no errors. The data link layer takes the bits of information and puts them into larger blocks of data called frames.
Then the network layer will make sure the data is in the correct order when it is transported. With a packet filtering router you can set up certain criteria the packets must meet to be accepted on the network. If the packets do not meet the set criteria then the data will be dropped.
A firewall will check traffic all the way up to the application layer. A firewall can be set up to allow some data to be transferred into the network or block it all. It can be set up to let some data leave a company as long as the destination meets the specification.
A firewall will examine the information contained in the header of a packet of information that is trying to travel over the network. Information checked includes the source address, the destination and the application it is being sent to. A firewall works on the network level of the OSI reference model.
Reference
Merkow, M. S., & Breithaupt, J. (2006). Information Security: Principles and practices. Upper Saddle River, New Jersey: Person Education INC. Retrieved May 21, 2010
