The opposite of a multicast (information is transmitted to all clients on the network), a unicast is a client-to-client communications exchange between a single requester and a single sender. If a malicious software can manage to send a duplicate unicast to every client on a network, this means that a separate stream for each session will occur at the same time, causing extreme network slowdown and possibly even a network crash. This method can therefore be exploited to create major bandwidth issues and service interruptions.
An authentication device that plugs into a USB port, adding an additional layer of access security to a system. The token might contain some type of security certificate or code that allows a user to access the system or login using the proper credentials.
Virtual Private Network (VPN)
A type of network that is a sub-set or "virtual" version of a physical network. Many businesses offer their employees access to a VPN as a means of securely connect to the business network from a remote location. This comes in handy for individuals on business trips or working from home. By using hardware such as proprietary network routers installed in their home or with the use of token-based authentication, a remote user can connect to their business network via the VPN which essentially creates a secure tunnel giving access to data normally only available when logging onto the network within the physical office (i.e. email systems, shared network drives, intranet pages, etc.)
The term virus is often over-used or misused for various types of malware. The term essentially refers to any type of malicious file or code that replicates itself (like a biological virus). A common misconception is that viruses are executed on their own. This is not correct. A computer virus is usually a self-contained program that will attach itself to an existing program. When a user runs that program, the virus, does whatever damage it was designed to do and then attaches itself to another program, which is eventually run, causing yet another program to be infected, and so on.
A system vulnerability is a weak spot in an application or operating system. A loop-hole if you will that can be exploited allowing malicious software or individuals to gain access to a system. Security patches and updates released by your operating system manufacturer are designed to "harden" vulnerabilities.