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I want to send one packet to the broadcast address, but wait for all responses.

If I do ping -c 1 192.168.1.255, it sends just one packet, but it quits after getting the first response.

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What do you want to do with that command? – Nils Feb 9 '12 at 21:24
@Nils: I just want a quick way to see approximately how many hosts are connected to the network. I know about nmap, but it has been a bit slow in my experience, compared to ping. – houbysoft Feb 9 '12 at 22:17
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Which version of ping? Even on Linux, there are several around. – Gilles Feb 9 '12 at 22:24
@Gilles: this is the standard ping on Mac OS X 10.7.3. I can't seem to find a switch for it to tell its version (tried -v, --version, searching the man page...). – houbysoft Feb 9 '12 at 22:47
Some versions of ping have a -w option to specify a length of time to wait, regardless of how many reply packets have been received. – jw013 Feb 10 '12 at 4:43
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2 Answers

up vote 3 down vote

How would ping know that the responses it got really are all of them?

To find out which hosts on a subnet are up, something like nmap is well suited. See the -sn argument for Host Discovery:

-sn (No port scan)

This option tells Nmap not to do a port scan after host discovery, and only print out the available hosts that responded to the scan. This is often known as a “ping scan”,[...]

Systems administrators often find this option valuable as well. It can easily be used to count available machines on a network or monitor server availability. This is often called a ping sweep, and is more reliable than pinging the broadcast address because many hosts do not reply to broadcast queries.

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I don't really care about "all of them", I just need most. Hence I can ping the broadcast and wait for the responses. I understand that ping has no way of knowing when will all responses arrive, but isn't there a way to specify a time for which it should wait? – houbysoft Feb 9 '12 at 20:44
For example, I can almost achieve what I want by "ping -c2 -i 10 192.168.1.255". It pings the broadcast and waits 10 seconds for the responses. However, in the end it sends another packet. Is there a way to make it wait 10 seconds, but not send anything afterwards? – houbysoft Feb 9 '12 at 20:45
up vote 0 down vote

This quick check for hosts possibly fails nowadays. There is an increasing number of operating systems that blocks incoming ICMP/ping by default. Perhaps there are better - and possibly faster ways to achieve your goal.

If you have SNMP-read-access to the involved router(s) you might get the information (number of hosts in a network) by a simple SNMP-get-request of the corresponding traffic statistic table.

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Is there a standard command that can be used to query this SNMP? – houbysoft Feb 11 '12 at 22:54
Here is a pack of free tools that help to analyse this data. – Nils Feb 12 '12 at 21:30
Blocking incomming ICMP is a symptom of braindamaged network admin. What most routers do nowadays is to just drop any network packet destined to the broadcast address (that was a popular DoS: send something that gives a sizeable response to the broadcast address forging the victim's address and watch it getting bogged down). – vonbrand Jan 18 at 0:29
@houbysoft sorry - I forgot to address you directly in my comment. – Nils Jan 19 at 21:33

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