Computers each have a unique address on a wifi network. This is an IP address. It's a specific location that identifies your computer on the network so that the information you are requesting on the Internet knows where to find you. If you don't have an IP address the Internet 'won't work' on your computer.
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Your IP address locates you and your activities on the Internet. This is how companies in one country control who can view their websites. Many only want their sites for people inside their own country. This is why sites such as Hulu, Netflix or television stations cannot be accessed from other countries -- like in Singapore.
Masking your IP address, in effect, will 'hide' you. Websites and networks won't know where you are from and your Internet traffic is coming from or going to.
IP masking sounds pretty good. You can access sites that were blocked. However, there are many consequences to this behavior.
Subversive behavior: Choosing to mask your IP address is being subversive -- it's trying to avoid the laws and rules that we are supposed to be following.
Breaking user agreements: Services that use IP addresses to limit the availability of their products have users acknowledge User Agreements that lay out actions such as IP masking are not allowed.
Legal risks: IP masking may be considered illegal in some countries and by some providers.
Ethical: Even if masking is not considered technically illegal, it could very well be considered unethical. Does a good digital citizen attempt to hide who they are and what they are doing? These questions are open for debate. Doubt means don't.
Most software that is 'free' is not. This is especially true for the most popular free IP masking software -- Hotspot Shield. Known and reported issues with installing this software include:
Advertisements in your browser
Slows down your browser
Trojan keystroke logger to record what you type
Add-on apps that are very hard to uninstall
Spyware
Reroutes your Internet traffic to third-party sites that can collect your browsing information