Many Internet Protocol (IP) applications assume that direct IP connectivity exists between hosts. In today’s Internet or extranets, this is often not true. The problems of limited IP address space have caused many enterprises to use private Request for Comment (RFC) 1918 addresses. These addresses cannot be routed and, for enterprises to connect to the Internet or to communicate in an extranet, address translations or application proxies must be used. For applications that exchange their IP addresses between the client and the server, these “dirty” IP address are not valid when one or both of the end systems exist on an RFC 1918 network. In addition, without using special techniques, applications like File Transfer Protocol (FTP) will not work when an enterprise uses private RFC 1918 addresses.
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