9
  • Host address: 192.168.0.13
  • Network: 192.168.0.0 255.255.255.0
  • Router: Cisco DPC3825 DOCSIS 3.0 Gateway (user manual)

My home network uses the 192.168.0.0 /24 network for my (mostly wireless) devices. On many other networks I frequent, I'll ping the applicable broadcast address to show all currently live devices in my (Windows 7 x64) results of arp -a. Pinging the broadcast address of a network helps me to identify devices that are currently communicating on the same network.

In my home network, I'm losing 100% of the packets sent to 192.168.0.255. Thus arp -a does not show me all of the devices that are live on my network. All devices connect directly to my router and then out to my ISP.

Why am I unable to ping the broadcast address in this network? Logging into the router directly to check the DHCP Client Table is inconvenient.

Update: These ping attempts are made from a computer directly connected to the router via ethernet. I've attempted the same ping attempts from multiple wireless devices to no avail.

That Brazilian Guy
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root
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3 Answers3

13

I think that the times when operating systems responded to broadcasts pings are long gone. As far as I know every modern operating system ignores those requests as a security measure to avoid broadcast storms.

The default in Linux:

$ sysctl net.ipv4.icmp_echo_ignore_broadcasts
net.ipv4.icmp_echo_ignore_broadcasts = 1

If you want to discover machines you'll have to resort to unicast ping (nmap, ping loop or other means), but note, there can be machines configured to always ignore ping requests.

Jorge Nerín
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4

It has already been discussed that answering pings to broadcast addresses is not considered good practice anymore. You could alternativly ping the all-host multicast group which uses the IP 224.0.0.1. In theory every multicast capable host should respond to a ping to this IP, but I've heard of hosts not doing so.

For more on multicast addresses read the tldp: http://www.tldp.org/HOWTO/Multicast-HOWTO-2.html

Another option would be to just unicast all IP addresses in your network segment which can easily be achieved by using built-in tools on most systems. I only really know GNU/Linux and MS Windows, so I can only give you examples for those systems.

GNU/Linux

for lastoctet in $(seq 254); do ping -c 1 192.168.0.$lastoctet; done

MS Windows

for /l %i in (1,1,254) do ping -n 1 192.168.0.%i

Not as easy as ping 192.168.0.255 but it does work under most circumstances.

bjanssen
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0

With reference to what #Jorge Nerin said:

Set the ignore parameter to 0. ie;

$ sysctl net.ipv4.icmp_echo_ignore_broadcasts
net.ipv4.icmp_echo_ignore_broadcasts=0

Do this with root privilege on all devices in the network. It will allow responses to broadcast pings.

PS : doing this may pose a security threat.

Flup
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Nithin Kumar
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