Solved: Broadcast Ping Question

cbrun · ‎04-24-2003

I have an 2524 router that has 2 subnets on the Ethernet interface. 172.18.19.0/24 primary IP address & 172.18.20.0/24 secondary.

I was looking for all the devices on the 172.18.20.0 subnet, so I did a broadcast ping (ping 172.18.20.255). The response I got was from every device on both 172.18.19.0 & 172.18.20.0. I expected to only get a reply from the devices addressed with 172.18.20.x.

Can someone help me with an explaination as to why the 172.18.19.x devices replied to a ping to 172.18.20.255?

Thanks

olorunloba · ‎04-24-2003

What to note is that the subnetting of the IP address on an interface does not change the broadcast address used. It is a funny one, but the thing is that the broadcast address on the interfaces is 255.255.255.255 except when changed using the ip broadcast-adress command. In pinging the subnet then (the 172.18.20.0), the router uses 255.255.255.255 broadcast address and not 172.18.20.255 broadcast address. Hence, the 172.18.19.0 subnet devices reply to this broadcast. If you change the broadcast address, you will see that they will not respond, but this might not be advisable in your situation.

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olorunloba · ‎04-24-2003

What to note is that the subnetting of the IP address on an interface does not change the broadcast address used. It is a funny one, but the thing is that the broadcast address on the interfaces is 255.255.255.255 except when changed using the ip broadcast-adress command. In pinging the subnet then (the 172.18.20.0), the router uses 255.255.255.255 broadcast address and not 172.18.20.255 broadcast address. Hence, the 172.18.19.0 subnet devices reply to this broadcast. If you change the broadcast address, you will see that they will not respond, but this might not be advisable in your situation.