The IP standard defines two types of IP addresses: Public and private. A public IP address is valid worldwide, while a private IP address only applies within a closed LAN.
Public IP addresses must be unique on a worldwide basis. Private IP addresses can occur any number of times worldwide; they must only be unique within their own closed network.
Normally, PCs in a LAN only have private IP addresses, while the router to the Internet also has a public address. All PCs behind this router have access to the Internet via its public IP address (IP masquerading). In such a case, only the router itself is responsive via the Internet. PCs behind the router are not responsive to the Internet without intervention by the router.
