What is IP address '::1'?

Question

I was playing with sockets on local machine with no network connection. See below:

IPAddress address = IPAddress.Any; // doesn't work
IPAddress address = IPAddress.Parse("::1"); // works

So what is exactly ::1 IP address ? Is it the default available IP address or it's the loopback address ? what happens to above code (working line) on a machine with dedicated IP address and network connection ?

EDIT:

exact code is used to bind a specific IP address to socket. Here it is:

ServicePoint sp = ServicePointManager.FindServicePoint(uri);
sp.BindIPEndPointDelegate = new BindIPEndPoint(Bind);
// here's the bind delegate:
private IPEndPoint Bind(ServicePoint sp, IPEndPoint ep, int retryCount)
{
   return new IPEndPoint(IPAddress.Parse("::1"), 0);
}

`IPAddress.Any` is `::0`, You should use `IPAddress.Loopback` for local (loopback) connection. — J-16 SDiZ, Jan 06 '11 at 03:31
I'm assuming the part that says `IPAddress.Pars` is really `IPAddress.Parse`, correct? — Brad, Jan 06 '11 at 04:21
[Reverse question](https://stackoverflow.com/questions/40189084/what-is-ipv6-for-localhost-and-0-0-0-0?noredirect=1&lq=1). — user202729, May 12 '18 at 04:56

Brad · Accepted Answer · 2013-03-27T21:25:14.220

205

::1 is the loopback address in IPv6. Think of it as the IPv6 version of 127.0.0.1.

See http://en.wikipedia.org/wiki/Localhost

edited Mar 27 '13 at 21:25

answered Jan 06 '11 at 03:18

Brad

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at above example `IPAddress.Parse("127.0.0.1")` doesn't work on my machine. – Xaqron Jan 06 '11 at 03:19
1

@Xaqron - that sounds more like a superuser question, because it probably means something is broken with your IPv4 TCP/IP stack. – Joel Coehoorn Jan 06 '11 at 03:21
Xaqron, maybe a firewall is blocking v4 but not v6 traffic? – SilverbackNet Jan 06 '11 at 03:25
I use `Windows Server 2008 Enterprise Edition 64-bit` and no third-party firewall installed. I'm wondering why I cannot bind `127.0.0.1` to my socket while `::1` is available. – Xaqron Jan 06 '11 at 03:30
@Xaqron, can you show us your code that you are trying to use? You say "works" and "doesn't work" but that isn't helpful to us. – Brad Jan 06 '11 at 03:37

score 31 · Answer 2 · answered Jun 14 '17 at 11:49

Just to add little more info to it, in IPv6 loopback address is represented as 127 zeroes followed by a 1 i.e (0000... 127 times..1). It's representation should have been like this -> 0000:0000:0000:0000:0000:0000:0000:0001 but we have some short form representation for this. If there are all zeroes in a single block you can replace it by single 0. So it becomes -> 0:0:0:0:0:0:0:0001. Again we can see that we have runs of zeroes, they can be eliminated and we get -> ::0001 -> ::1 .

(to be clear, the "127 zeroes" here are in binary, not hexadecimal) — user202729, May 12 '18 at 04:57

score 3 · Answer 3 · answered Dec 22 '21 at 09:41

3

The simple answer is that: ::1 is the compressed format of IPV6 loopback address 0:0:0:0:0:0:0:1. It is the equivalent of the IPV4 address 127.0. 0.1

answered Dec 22 '21 at 09:41

Ali Mumtaz

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What is IP address '::1'?

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