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 ava开发者_如何学JAVAilable xlib.crc32)

Thanks!

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Why do you want to hash an IP address into a number? They already have a native integer representation. For example, using netaddr:

>>> import netaddr
>>> ip = netaddr.IPAddress('192.168.1.1')
>>> ip.value
3232235777
>>> netaddr.IPAddress(3232235777)
IPAddress('192.168.1.1')

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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.

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ad 1) It will yield different results, but does not effect the quality of the hash.

ad 2) It will always yield a positive number or zero.

ad 3) As you limit the number of possible buckets, it does affect the quality of the hash.

In general: About how large is your H? Remember that a IPv4 address is nothing more than a 32-bit value. 192.168.0.1 is just a more human readable byte-wise representation. So if your H is larger than 4294967295, there will be no need of hashing.