What is IPV6 for localhost and 0.0.0.0?

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:

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