将IP地址映射到阵列索引

Question

I am looking for a good way to map IP addresses to indexes in an array. Say I have an array of 32 size and maximum possible IP addresses which we can get as input from user will always be <= 32. I would like to map this IP addresses to some index position in the array and from thereon use this index position to refer to this IP address.

What would be the best technique for doing this. IP address is essentially just an integer. Collisions also need to be handled if two addresses map to the same index position.

ip1 => convert to some integer say i1 => index is now just i1 % 32

Any hashing technique which fits well into this use-case to convert the IP address would also be useful.

Thanks for your help.

Answer 1

Are you expecting any hashing algorithm reference for your problem?

Since you have array size 32.

You can just use number of set bits in you IP address to index to the array location. But all 0 and all 1 are not used addresses so you are at loss of 2 locations. You can check using sum of all the nibbles ignoring the carry bits for your hashing. You can increase number of bits added based on your array size.

you might also base your hash on only the 2 bytes of your IP address since the higher order bytes may be same in a single sub-net.

For case of collision you would need to have a pointer array instead of a single dimensional array so you can store multiple addresses with same hash.

Answer 2

There cannot be a hash function that makes this possible. The hash function takes the ip address only as a param, while you need knowledge of the other elements to accomplish what you want: if you have 32 positions and > 32 possible values for your elements, there must be collisions for any hashfunction.

If this was possible, hashmap implementations would not have to consider collisions until the size would be bigger than the max size of the underlying array. And they do consider that :)

It might be helpfull to see how java hashmap maps a hashcode to an index of the underlying array:

static int  indexFor(int h, int length) {
     return h & (length-1);
 }

If you use this, keep in mind to use powers of 2 to make this effective. Java starts with 16 and increases to 32, 64 etc.