Using Internet technology inside corporate network is referred to as intranet.
IP addresses must be assigned uniquely worldwide. Just a few years ago, many companies had their networks built on the TCP/IP protocol and no one had ever dreamt of connecting them to the Internet. This is why they chose entirely arbitrary addresses for their networks. Nowadays, companies tend to connect their networks to the Internet through a firewall and some discover that their addresses are already used by someone else. They are forced to renumber their networks, which is a painstaking operation.
At first, companies using intranet addresses colliding with Internet ones mostly tried to find a fresh solution to avoid renumbering their intranet addresses. As a solution we mention Network Address Translation (NAT) , but solutions of this kind have drawbacks of their own and these vain efforts are usually followed by renumbering the whole intranet anyway...