Sites are usually blocked by some point in a user’s network. For example, if a user is connecting to the Internet through a place at work or through a college, these institutions often monitor traffic. The network knows what a user’s IP address is, and it specifies that those IP addresses may not connect to certain sites. Regular Internet service providers or even national governments may also use this technique to prevent users from connecting to restricted sites. An example is the “Great Firewall of China” that specifies that no IP address that comes from areas within China may connect to certain websites deemed to be restricted by the government, such as sites that contain criticism against the Chinese government.
Proxy sites and VPN programs bypass this block by filtering a user’s IP address through a different IP address that isn’t within the restricted range. For example, if a person in China uses a VPN to make it look like they have an address from the United States, the Chinese Firewall won’t block it.Learn more about Internet & Networking