Since the early 1990s, the concern about the depletion of IPv4 address space has been a priority of the IETF. The combination of RFC 1918 private IPv4 addresses and NAT has been instrumental in slowing this depletion. NAT has significant disadvantages, and in January of 2011, IANA allocated the last of its IPv4 addresses to RIRs.
One of the unintentional benefits of NAT for IPv4 is that it hides the private network from the public Internet, as shown in the figure. NAT has the advantage of providing a perceived level of security by denying computers in the public Internet from accessing internal hosts. However, it should not be considered a substitute for proper network security, such as that provided by a firewall.
In RFC 5902, the Internet Architecture Board (IAB) included the following quote concerning IPv6 network address translation:
“It is commonly perceived that a NAT box provides one level of protection because external hosts cannot directly initiate communication with hosts behind a NAT. However, one should not confuse NAT boxes with firewalls. As discussed Section 2.2 in RFC4864, the act of translation does not provide security in itself. The stateful filtering function can provide the same level of protection without requiring a translation function.”
IPv6, with a 128-bit address, provides 340 undecillion addresses. Therefore, address space is not an issue. IPv6 was developed with the intention of making NAT for IPv4 with its translation between public and private IPv4 addresses unnecessary. However, IPv6 does implement a form of NAT. IPv6 includes both its own IPv6 private address space and NAT, which are implemented differently than they are for IPv4.