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Unicast filter for NAI Distributed Sniffer 1
- Thread starter ashari1
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Sniff4Food
Instructor
Simple. Do a binary pattern match for the last bit in the first byte of the Destination Address (the first byte in the packet). If the bit is a 0 the packet is a unicast, if it is a 1, it is either a broadcast or multicast.
More detail than you wanted to know: Ethernet transmits the bits onto the wire in reverse bit order so the last bit in the byte is really the first received. So, if the bit is a 1 and you don't support multicast, the first 0 you see, you know the address is multicast and you don't have to copy it into your buffer (all 1s would be broadcast). A quick way to tell if an address is uni, multi or broad is to look at the first byte. If it is odd, it is a multicast. If it is even, it is a unicast. If it is FF (all binary 1s), then it is a broadcast.
More detail than you wanted to know: Ethernet transmits the bits onto the wire in reverse bit order so the last bit in the byte is really the first received. So, if the bit is a 1 and you don't support multicast, the first 0 you see, you know the address is multicast and you don't have to copy it into your buffer (all 1s would be broadcast). A quick way to tell if an address is uni, multi or broad is to look at the first byte. If it is odd, it is a multicast. If it is even, it is a unicast. If it is FF (all binary 1s), then it is a broadcast.
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