Pinging a broadcast address

Dist · 13 Nov 2007 at 04:21

Is pinging a broadcast IP not possible? I ask because i was reading/watching about subnets on quite a decent CCNA program (testout navigator), and after working out what the usable ips are when given an ip and subnet, i decided to try it on myself, and then ping the broadcast address and get multiple replys back if i was right.

Well, i checked my router to see what IP and subnet mask i have, and i have /23 subnet, so from that i figured out the address range 82.45.102.0 - 82.45.103.255, and tried pinging the last address to see if i would get multiple responses (i thought if the broadcast address sends the pings to the other 509 ips in that range, then i would get lots of responses confirming my workings out). But i got nothing.

So are broadcast addresses not pingable? or is it the hosts in the network that just dont respond to a ping if its sent to the broadcast address instead of directly to them?

sidethink · 13 Nov 2007 at 08:34

Pings to the broadcast address should in theory return results but since they are also used to create packet storms and DOS attacks, most networks silently drop any pings to their broadcast addresses - this is specifically catered for in the specification for the protocol.

tolien · 13 Nov 2007 at 14:05

Unless you own that entire /23, the ISP probably blocks pings to the broadcast address for the reasons sidethink mentioned.

mynsa · 13 Nov 2007 at 16:43

in linux you can ping a broadcast address by specifying the -b tag, you can do this on your local network, as you control that.

for the 10.0.0.0/24 subnet,

ping 10.0.0.255 -b

however over the isps network, they will probably drop the packets

Dist · 13 Nov 2007 at 16:50

tolien said:
Unless you own that entire /23, the ISP probably blocks pings to the broadcast address for the reasons sidetink mentioned.

I only own a single IP. So it must be the ISP blocking the ping. Im supprised that by default my LAN would drop broadcasted pings though, my netgear router and built in ethernet on my 5 year old motherboard isnt that advanced, i thought only newer hardware would be sensible enough to drop broadcasted pings by default.

At least that means i wasnt wrong though, i thought that one reason why i got no ping response could be that the IP i pinged was just some random IP thats not inuse, and wasnt actully the broadcast. Now i know it proberly is the broadcast IP, but just not used.

Thanks for the responses everyone.