Why the ping to IP 192.168.3.255 got reply from another host on same subnet?

[Ranjeet_2]

Why the one device (Access Point or Switch) connected on the same subnet replies the ping request to 192.168.3.255 send by another host on the same subnet.

The environment is as following IP/Subnet Mask

1. Connect one network Device (192.168.3.100/255.255.255.0) on hub.
2. Connect one PC (192.168.3.190) on same hub.
3. Now ping from PC to IP address 192.168.3.255 and capture the ethernet packets.

The network device wills response to PC ping request.

Why it happens? Please tell me (raini@punjabi.net ) is this Device problem or Device behavior?

[Antoniov.]

No, is a right answer.
Address xxx.xxx.xxx.255 are called broadcast address; this means every host can reply to this request.
Your Switch answer to broadcast ping and this means it work fine!

H.T.H.
Antonio Vigliotti

[Lokesh_2]

Hi Ranjit,

You are getting reply because the address 192.168.3.255 is broadcast address. Any system, PC , network device..etc on the LAN will accept this IP and will reply back to ping. This is not a bug or problem !!

HTH,
Thanks & regards,
Lokesh Jain

[Ranjeet_2]

Hi!
Antonio Vigliotti and Lokesh,

Thanks for your reply.
Antonio, As you said that every host can reply to the request. But its not true. Becasue if one the same hub there is not Switch (only two or three PCs), DHCP server is there, it can listen the broadcast also. But it don't reply to that ping broadcast. Only if the switch or AP is there they used to reply back. If we capture the packets and we can find that who's network device on the network. Any body can know the address of that network device. Switch should block that broadcast request. How do u think?
------
Lokesh, As you said that "Any system, PC , network device..etc on the LAN will accept this IP and will reply back to ping". No if we use 3 PCs (One XP, one windows2003 server and one win2000 prof). 2003 Server's DHCP server is there, it can listen the broadcast also. But it don't reply to that ping broadcast. So why only Switch or AP ?
(Here is have attached ping packet captured using Ethereal)

[Mike Naime]

Ranjeet:

I'm sorry if you do not like the answer that you received. If you want a better explanation than .255 is a broadcast address. I would recommend that you ask that question on the networking forum, not the VMS forum. While there is a lot of knowledge here, this is not the IP networking forum.

When you have a network on even a HUB, a BROADCAST message goes out to all of the devices. Not all of the devices may respond to that broadcast, but they are indirectly addressed by it.

Basically, you are yelling out "IS ANYBODY OUT THERE?" any someone is responding "YES" to you.

Here is what I get on my home network of 5 PC's

C:\WINDOWS\Desktop>ping 192.168.1.255

Pinging 192.168.1.255 with 32 bytes of data:

Reply from 192.168.1.255: bytes=32 time=2ms TTL=255
Reply from 192.168.1.255: bytes=32 time<10ms TTL=255
Reply from 192.168.1.255: bytes=32 time<10ms TTL=255
Reply from 192.168.1.255: bytes=32 time=1ms TTL=255

Ping statistics for 192.168.1.255:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 2ms, Average = 0ms

C:\WINDOWS\Desktop>


Mike

[Martin P.J. Zinser]

As noted before, this is expected behaviour. If some of your PCs do not answer to this ping this is rather an issue of these PCs and not of thie request. Pinging a broadcast address is a supported feature. Check ping -b on a Linux system for example.

Greetings, Martin

[Antoniov.]

Hi Ranjeet,
in tcp/ip standard broadcast address is the address with all 1 bits in host field; in your example, address mask is 255.255.255.0 so broadcast address is xxx.xxx.xxx.255 where xxx.xxx.xxx is network address (192.168.3.0).
When a device send a broadcast socket any device in network can anser (no must answer!); usually broadcast socket is use to configure device (DHCP request, gateway address request, etc.); this means PC that has nothing to tell doesn't answer to broadcast. As all socket type, also broadcast socket has service request to specify the type of service request.
I think your device work fine and you haven't problem.

Antonio Vigliotti

[Ranjeet_2]

Thanks everybody for reply by you for submited question by me. Actually some asked me about this question and he is the member of IEEE. May be later when i would get his response, then i will put here again.

Thanks all.

[Lokesh_2]

Hi Ranjeet,

Sorry if you have already tried this. Try to ping your DHCP server and see if you are getting any response.

Thanks & regards,
Lokesh