[1][2] Most Internet service providers (ISPs) allocate only a single publicly routable IPv4 address to each residential customer, but many homes have more than one computer, smartphone, or other Internet-connected device.
In this situation, a network address translator (NAT/PAT) gateway is usually used to provide Internet connectivity to multiple hosts.
Private addresses are also commonly used in corporate networks which, for security reasons, are not connected directly to the Internet.
Often a proxy, SOCKS gateway, or similar devices are used to provide restricted Internet access to network-internal users.
[4] The concept of private networks has been extended in the next generation of the Internet Protocol, IPv6, and special address blocks are reserved.
[2] They are unicast addresses, but contain a 40-bit random number in the routing prefix to prevent collisions when two private networks are interconnected.
Examples: A former standard proposed the use of site-local addresses in the fec0::/10 block, but because of scalability concerns and poor definition of what constitutes a site, its use has been deprecated since September 2004.
The validity of link-local addresses is limited to a single link; e.g. to all computers connected to a switch, or to one wireless network.
The AS112 project attempted to mitigate this load by providing special black hole anycast nameservers for private address ranges which only return negative result codes (not found) for these queries.