Thanks very very much I am getting it now. Kept reading what you stated -- this was a lie in fact and the truth follows I was stuck on multiplying by 4.....8 bits in a byte I should still be dividing to get bytes not multiplying then ad nauseum I think I hit what you explicitly put :) units of 32 bits ... 32/1*1/8=4 everyone of these words is 4 bytes so whatever number of words I have I multiply by 4, like you said then 2^2 left shift.... like you said right 2 -> shift 6 :) thanks much again! thanks MUCH again On Mon, May 21, 2018 at 6:22 PM, Andy Furniss <adf.lists@xxxxxxxxx> wrote: > jsl6uy js16uy wrote: >> >> Hello all hope all is well >> >> I looked around and have tried to get this sorted on multiple >> occasions, but just can't get it >> how does dividing by 64/2^6, ~ shift 6, ultimately get you to >> something like the proper location for a src port in layer 4? >> I get, after reading again and again and looking at ip packet header >> diagrams, offset at 0 and the mask used to extract the IHL from the >> 16bit word. >> But then comes 'ol shift 6 ... right-shifting by 6 eliminates the >> offset of the field and at the same time converts the value into byte >> unit ....and I am lost again >> IHL # 32 bit words I guess dividing by 64 bits will get you bytes >> multiples 8 and then I have nothing >> >> googling around verified binary math and this is dividing by 64, but >> that's where it ends, anything else is people just using it. I figure >> I must be missing the deal. >> Would like to understand it to fully use > > > If you read the first 16 bits and mask to leave just IHL then you get > (assuming normal IHL = 5) > > 0000 0101 0000 0000 this is not 5 (101 in binary) to get 5 you would > need to right shift by 8 = 0000 0000 0000 0101. > > Your examples want IHL in bytes (octets) and since IHL is in units of > 32 bits you need to multiply by 4 to get bytes. > > left shift by 2 is x4 so instead of right shift 8, right shift 6 gives > the IHL x4 = bytes > > 0000 0000 0001 0100 = 20. > > Either way you know the header length and for tcp or udp the next 16 > bits will be the source port. -- To unsubscribe from this list: send the line "unsubscribe lartc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
