python - Simple operations with TCP/IP Header bits -



python - Simple operations with TCP/IP Header bits -

i have 2 doubts related operations bits:

the first 1 >> operator. far have read, operator this

0001 0000 >> 4 --> 0000 0001 or 1111 0001, not sure if fills 1s or 0s

so have code:

version_ihl = iph[0] version = version_ihl >> 4 ihl = version_ihl & 0xf

let's ip version 4: 0100 , ihl 5: 0101

if version = version_ihl >> 4, result be

0100 0101 ---> 0000 0100 in case operator >> doesn't fill 1, have read does, instead of ip version = 4 1111 0100 not 4 @ all.

the sec thing line, how calculate ihl ihl = version_ihl & 0xf

the final result 5, 0000 0101 , far know, operator mask of 1s, 0100 0101 remain same. 2 questions :

1.how obtain 0100 0101 >> 4 operation result 0000 0100 if >> supposed fill 1s¿?

2.how obtain 0100 0101 result 0000 0101 & 0xf mask¿?

the code posted worked right, solution there, want know why works that. give thanks in advance, hope through lite on this.

the >> operator not "fill" anything. integers in python behave though have infinite number of leading zeros (nonnegative integers) or ones (negative integers). right-shifting drops n to the lowest degree important bits. 0xf same 0b1111, in turn same 0b00001111. masking to the lowest degree important nybble ones.

since asked more detail:

you started 0100 0101 , right-shifted four. that's exactly equivalent starting 0000 0100 0101 , right-shifting four, gives 0000 0100. that, in turn, same 0100. you started 0100 0101 , masked 0xf. 0xf 1111, same 0000 1111. 0000 0101, or 0101

python bit-manipulation bit-shift tcp-ip raw-sockets

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