Describe the physical and data link layers of a network.
The physical layer is composed of the electronic signals transmitted over fiber, coaxial cable, twisted pair wires, modem lines and infrared. The physical layer is always present whenever data is transmitted over any medium. The data link layer defines how the signals from the physical layer are reassembled into a usable format.

To maintain security in these layers, you must understand the network topology^[1] that you are protecting. Common hacker methods for attacking and penetrating a network are to install a packet sniffer^[2] onto one of the company's internal machines or to attach a wiretap to the physical media. Using the techniques of encryption^[3], data labels, and traffic padding make it very difficult for a hacker to successfully use any information obtained from a sniffer.

The protocols used at the network layers assist in host-to-host communication. Most security threats do not originate at this layer of the TCP/IP stack; however, you must know what protocols are used.

[1]Network topology :The type of network (ethernet or token ring), the IP address range, the subnet mask, and the naming scheme. The most common network topologies are the star, bus, ring and hybrid.

[2]Packet sniffer: A device or program that is used to monitor traffic on a network, can be installed anywhere in a networked system, and is virtually undetectable. Sniffers are used for legitimate network management functions or for stealing information off a network.

[3]Encryption: The process of disguising a message to make it unreadable by humans. The resulting data is called ciphertext.