Private IP addresses belong to certain IP address ranges that are not routed on the Internet. They can be used by anyone for private networks such as LANs.
The idea to set private address ranges, arose from the desire, without unnecessary administrative overhead to maintain local networks. As part of the emerging shortage of IP addresses on the Internet Protocol v4, this was overlaid by the requirement, as far as possible to save public IP addresses.
Many computer networks require full connectivity among themselves while at the IP level, but only a limited Internet access. Tells you to such a computer network a private IP address range, it is an intranet, on the from the Internet can not be accessed because the Internet routers ignore the private address ranges. A gateway or router that is placed in this private network, and next to the private IP address also has a public address of the Internet, makes the Internet. This can be done through a proxy or using NAT / PAT / Masquerading. Since the address range used visible only within the private network is, the same area and in other private networks are used. The uniqueness of the Internet Protocol required an IP address is maintained by the limited visibility. As a side effect of the direct access will be blocked from outside the local clients in the private network, which brings but compared with a correctly configured firewall no security benefit. Only the disguise of the local IP addresses can be seen as an improvement of privacy.
Of the three IANA private IP address ranges have been established, with each of the three areas located in a different class to the historical net classes concept. The address ranges have been documented in RFC 1994 1597th This RFC was superseded in 1996 by the then still valid RFC 1918, wherein the private address ranges are not modified.
In a typical private network, all computers are assigned with IP addresses of 192.168.1.x. The IP address of router is commonly set to 192.168.1.1, and the subnet mask is set to 255.255.255.0. Default gateway should have the same address with the router, which is 192.168.1.1.