Passage: The successor to IPv4 is IPv6. IPv6 was a result of several years of experimentation and dialog during which various protocol models were proposed, such as TP/IX (RFC 1475), PIP (RFC 1621) and TUBA (TCP and UDP with Bigger Addresses, RFC 1347). Its most prominent difference from version 4 is the size of the addresses. While IPv4 uses 32 bits for addressing, yielding c. 4.3 billion (4.3×109) addresses, IPv6 uses 128-bit addresses providing ca. 340 undecillion, or 3.4×1038 addresses. Although adoption of IPv6 has been slow, as of June 2008, all United States government systems have demonstrated basic infrastructure support for IPv6.The assignment of the new protocol as IPv6 was uncertain until due diligence revealed that IPv6 had not yet been used previously. Other protocol proposals named IPv9 and IPv8 briefly surfaced, but had no affiliation with any international standards body, and have had no support. However, on April 1, 1994, the IETF published an April Fools' Day joke about IPv9.

Question: What was the successor of the IPv4?
IPv6