>Hi,
>
>I want to implement a hash routine for IP addresses. Basically
>if f(x) is my hash formula then i would want an efficient f(x)
>such that f(x) on most of the IP addresses gives me unique
>values so that i dont have collisions. I can have a limited
>array size (say 255) and i have to use this. Any clues.
>
>The formula that I have in my mind is simple. I will calculate
>the hash as (IP Address value as unsigned int) % HASH_SIZE,
>where HASH_SIZE is 255.
>
>Anybody aware of any better formula?
Not necessarily better. Depends on the variety of IP addresses you're
expecting, for example are they all likely to be in one of the 'local'
ranges? (10.0.0.1 .. 10.255.255.254, 172.16.0.1 .. 172.31.255.254 or
192.168.0.1 .. 192.168.255.254) or global?
Anyway, assuming a prototype of
unsigned char f(unsigned long ipaddress){
There are a few ways you could do it:
return ipaddress % 255; /* your method */
return ipaddress & 0xff; /* last octect of IP address -
slightly different, one of 256*/
return (ipaddress &0xff) ^
((ipaddress >> 8) &0xff) ^
((ipaddress >> 16) &0xff) ^
((ipaddress >> 24) &0xff); /* XOR all
bytes of address - again, 0-255*/
srand(ipaddress);
return rand() / (RAND_MAX / 256 + 1); /* plain weird */
--
PJH
"Consistently separating words by spaces became a general custom about
the tenth century A.D., and lasted until about 1957, when FORTRAN
abandoned the practice." -- Sun FORTRAN Reference Manual
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