IANA tracks IPv4 address space in blocks of 16 million addresses, which are known in network engineering parlance as a “/8.” At the beginning of 2010, IANA had 26 /8s left.

IPv4 dates back to 1981 and only has room for 4.3 billion unique addresses. IPv6, introduced in 1999, should have enough addresses to serve Internet users for generations, according to ARIN.