What is IPV6 for localhost and 0.0.0.0?

As we all know the IPv4 address for localhost is 127.0.0.1 (loopback address). What is the IPv6 address for localhost and for 0.0.0.0 as I need to block some ad hosts.

As we all know that IPv4 address for localhost is 127.0.0.1 (loopback address).

Actually, any IPv4 address in 127.0.0.0/8 is a loopback address.

In IPv6, the direct analog of the loopback range is ::1/128. So ::1 (long form 0:0:0:0:0:0:0:1) is the one and only IPv6 loopback address.

^{While the hostname localhost will normally resolve to 127.0.0.1 or ::1, I have seen cases where someone has bound it to an IP address that is not a loopback address. This is a bit crazy ... but sometimes people do it.}

^{I say "this is crazy" because you are liable to break applications assumptions by doing this; e.g. an application may attempt to do a reverse lookup on the loopback IP and not get the expected result. In the worst case, an application may end up sending sensitive traffic over an insecure network by accident ... though you probably need to make other mistakes as well to "achieve" that.}

Blocking 0.0.0.0 makes no sense. In IPv4 it is never routed. The equivalent in IPv6 is the :: address (long form 0:0:0:0:0:0:0:0) ... which is also never routed.

The 0.0.0.0 and :: addresses are reserved to mean "any address". So, for example a program that is providing a web service may bind to 0.0.0.0 port 80 to accept HTTP connections via any of the host's IPv4 addresses. These addresses are not valid as a source or destination address for an IP packet.

Finally, some comments were asking about ::/128 versus ::/0 versus ::.

What is this difference?

Strictly speaking, the first two are CIDR notation not IPv6 addresses. They are actually specifying a range of IP addresses. A CIDR consists of a IP address and an additional number that specifies the number of bits in a netmask. The two together specify a range of addresses; i.e. the set of addresses formed by ignoring the bits masked out of the given address.

So:

• :: means just the IPv6 address 0:0:0:0:0:0:0:0
• ::/128 means 0:0:0:0:0:0:0:0 with a netmask consisting of 128 bits. This gives a network range with exactly one address in it.
• ::/0 means 0:0:0:0:0:0:0:0 with a netmask consisting of 0 bits. This gives a network range with 2¹²⁸ addresses in it.; i.e. it is the entire IPv6 address space!

For more information, read the Wikipedia pages on IPv4 & IPv6 addresses, and CIDR notation:

• https://en.wikipedia.org/wiki/IPv4_address
• https://en.wikipedia.org/wiki/IPv6_address
• https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing

IPv6 localhost

::1 is the loopback address in IPv6.

Within URLs

Within a URL, use square brackets []:

• http://[::1]/
  Defaults to port 80.
• http://[::1]:80/
  Specify port.

Enclosing the IPv6 literal in square brackets for use in a URL is defined in RFC 2732 – Format for Literal IPv6 Addresses in URL's.