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pythonip-addresshashzlibcrc32

Hashing an IP address to a number in [0, H)

I'm using Python-2.6. I have very little knowledge of hash functions.

I want to use a CRC hash function to hash an IP address like '128.0.0.5' into the range [0, H). Currently I'm thinking of doing 

zlib.crc32('128.0.0.5')%H.

Is this okay? There's a few ques. you could try and answer...

  • does it make any diff. if I hash '128.0.0.5' or its binary '0001110101010..' whatever that is or without the '.'s

  • zlib.crc32 returns a signed integer. Does modding (%) a neg. with a positive H always give a pos no?

  • Does %-ing by H affect how good the hash function is? ( I mean is that the best I could do for the available space, with the available xlib.crc32)

Thanks!

Solution

  • does it make any diff. if I hash '128.0.0.5' or its binary '0001110101010..' whatever that is or without the '.'s

    Not really.

    zlib.crc32 returns a signed integer. Does modding (%) a neg. with a positive H always give a pos no?

    Yes.

    Does %-ing by H affect how good the hash function is? ( I mean is that the best I could do for the available space, with the available xlib.crc32)

    You'd better use all the bits of the checksum to make up for their lack of an "avalanche effect". Single-digit variations such as 192.168.1.1, 192.168.1.2, etc might produce differences only in the first bits of the checksum, and since % cares only about the last bits, hashes will collide.