I use this one all the time: 127.0.0.1 == 127.1 10.0.0.1 == 10.1 192.168.0.1 == ...

nwh · on Oct 20, 2013

I wasn't aware of that shortcut, that's very useful.

kbenson · on Oct 21, 2013

It's especially useful for some IPv6 addresses.

ordinary · on Oct 21, 2013

IPv6 has a different representation format, where one (and only one) sequence of fields containing only 0 bytes may be replaced by a single '::', turning '2001:0:0:1:0:2' into '2001::1:0:2'.

I never quite understood why they chose to reuse the colon as a separator character, because this is problematic when you want to append a port number. '2001::1:0:2:8080' could either be '2001:0:0:1:0:2 on port 8080', or '2001:0:1:0:2:8080 on the default port'. The following (ugly) syntax is used to resolve that ambiguity: [2001::1:0:2]:8080.

0xbadcafebee · on Oct 21, 2013

More IPv6 hackery: When using a link-local address (every interface has a link-local IPv6 address), you can append a zone index to the address in the brackets as routing information of where to send the link local request. For windows it's the interface number, for Unixes it's the interface name.

To test this out, first find a link-local address on your network. To find all link-local addresses on eth0, use this command to ping the all-nodes link-local multicast group:

  ping6 -I eth0 -c 2 ff02::1

Now append the percent sign and zone index to the address and connect to the host on a port that is probably open.

  lynx https://[fe80::21e:67ff:fe08:d500%eth0]:443/

SneManden · on Oct 21, 2013

I often use 0 == 0.0.0.0 == localhost

ClashTheBunny · on Oct 21, 2013

This is not true. 0.0.0.0 may end up at localhost for you, but it's not localhost. I just get: PING 0.0.0.0 (0.0.0.0): 56 data bytes ping: sendto: No route to host

Which system do you use that this works on?

reidrac · on Oct 21, 2013

I don't know about the OP, but in Ubuntu Precise "ping 0" pings to 127.0.0.1.

I can't explain why.

ordinary · on Oct 21, 2013

From what I can tell, the use of '0' or '0.0.0.0' as a destination address is not official. It should only be used as a source address. From RFC 5735 - Special USe IPv4 Addresses:

   0.0.0.0/8 - Addresses in this block refer to source hosts on "this"
   network.  Address 0.0.0.0/32 may be used as a source address for this
   host on this network; other addresses within 0.0.0.0/8 may be used to
   refer to specified hosts on this network.

Support for it looks to be flaky. 'ping 0' and 'ping 0.0.0.0' work for me on various Linuxes ("64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.026 ms"), but not on Windows 7 ("sendto: Cannot assign requested address") or Windows Server 2003 ("Destination specified is invalid").

reidrac · on Oct 21, 2013

IPv6 behaviour seems to be the same: "ping6 ::" pings to ::1: (localhost).

This is intriguing, really :)

EDIT: oh, you already pointed to the RFC :)

laumars · on Oct 21, 2013

My understanding is that 0.0.0.0 is the opposite of localhost as binding a daemon to 0 would bind that service to every interface and on any available IP. This means if you have MySQL (for example) listening on 0, it's not just listening on your localhost address but also on any WAN / LAN IPs you have as well.

pahans · on Oct 21, 2013

aha. i didnt know this. thanks