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Broadcast addresses

By shayan256 ·
Hi all,
I need some information on subnet broadcast addresses and the way computers on a network are supposed to behave when they receive broadcast messages.

This is what I?m trying to do:
I have 2 computers connected to each other. Computer 1 has IP 192.168.1.1 and computer 2 has IP 192.168.1.2. There is a PPP connection between these 2 computers. (computer 2 is actually a machine running VxWorks 6.0 and is utilizing VxWork?s PPP stack) Computer 2 is connected to a local network (via Ethernet) and its subnet mask is 255.255.255.0, so its broadcast address becomes 192.168.1.255.
When I try pining 192.168.1.255 from any machine on the same subnet (subnet 1) with the same net mask (255.255.255.0), every computer on the subnet that has 192.168.1.255 as its broadcast address responds (including computer 2). However, when I ping 192.168.1.255 from computer 1, broadcast ping packets never go out of Ethernet port that connects computer 2 to the network. I can verify this using Ethereal. This is weird because ARP table shows an entry after the ping attempt. The entry is192.168.2.255 -> 0xFFFFFFFF for the Ethernet interface of computer 2. I know that 0xFFFFFFFF is the default MAC address for broadcast so every NIC on the subnet should receive the packets. Can anyone tell me if broadcast packets are supposed to be forwarded from computer 2 to the network? If yes, why computer 2 "consumes" the packets?

Thanks for your time,

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by curlergirl In reply to Broadcast addresses

If your description of the ARP entry is correct, it looks to me as though the IP address of computer 2 is actually on a different subnet - 192.168.2.x instead of 192.168.1.x. Or perhaps this is a characteristic of the VxWorks software - I'm not at all familiar with VxWorks. Anyway, if the actual IP address of computer 2 is on a different subnet, then there must be a router between the two subnets, and routers do not normally forward IP broadcasts unless they are especially configured to do so.

Hope this helps!

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by shayan256 In reply to Broadcast addresses

Sorry for the misleading info. ARP entry is actually 192.168.1.255->0xFFFFFFFF. It was a typo.

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by shayan256 In reply to Broadcast addresses

Sorry for the misleading info. ARP entry is actually 192.168.1.255>0xFFFFFFFF. It was a typo.

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