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A ping to responds from However, if I shutdown the localhost loopback and keep another interface up with another ip, then ping gives an error

$ping connect: Invalid argument

Isn't supposed to listen to all interfaces? So basically, how exactly does a binding work from network perspective? how does ping work only for loop-back and not for other interface?

  • Update I have read it elsewhere and got similar answer here (you cannot ping

[anshup@s2 ~]$ ping
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=64 time=0.010 ms
64 bytes from icmp_seq=2 ttl=64 time=0.009 ms
64 bytes from icmp_seq=3 ttl=64 time=0.009 ms
^C --- ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2551ms
rtt min/avg/max/mdev = 0.009/0.009/0.010/0.002 ms

So what exactly is happening here if we cannot ping ? and re-iterating the question, why does response back only from loop-back and not any other interface on the host?

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2 Answers 2

up vote 12 down vote accepted is not an address but an network Identity (used for routing, and means 'All Networks'). as such you cannot ping it. if you wanted to ping every system you would use Every network has two special addresses, the Identity, and the Broadcast, usually at .0 and .255 respectively.

for instance on the network, the Identity =, and the broadcast = the first host is, and the last is note that no host may use the Identity or the Broadcast address as their own IP address.

the host default gateway is by definition the route you utilize to reach all unknown networks, so is just a reference to All Networks.

Edit: The reason the loopback is responding is 'undefined' behavior not specified in RFC1700. per the RFC, an address starting with 0 is never allowed as a destination (only as the source of a broadcast packet), so the author of your network stack choose to have the loopback respond. when I attempt to ping from a win7/powershell prompt, I simply get the message 'PING: transmit failed. General failure.' so this is clearly behavior specific to a given OS stack, or perhaps even driver-specific. If you provide your OS, we might be able to find more, but it would answer no meaningful questions since its implementation specific behavior not governed by networking standards. My guess is your OS is simply aliasing all invalid addresses as 127.x.y.z addresses.

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Thanks for explaining this! – SPRBRN May 7 '13 at 13:51
@frank: Thanks for answering. Updating the question to add more info in view of your answer. – Anshu Prateek May 8 '13 at 12:02
@Frank: Thanks for the update/edit answer. That helps. Am on fedora 17. Will check into its implementation. – Anshu Prateek May 9 '13 at 2:52

In the Internet Protocol version 4 the address is a non-routable meta-address used to designate an invalid, unknown or non applicable target.

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