How do I detect whether a given IP is in a given subnet in Javascript?

Question

I'd like to write a function in Javascript along the lines of:

in_subnet(ip, network, slash) {
    ...
}

in_subnet('1.2.3.4', '1.2.0.0', 16) # True, since it's in 1.2.0.0/16
in_subnet('1.2.3.4', '1.2.0.0', 24) # False, since it's not in 1.2.0.0/24

Should I write it from scratch, or are there some good libraries I could use? Or is the entire function already written and in the public domain?

Answer 1

Go ahead and learn it. :)

The trick is to parse '1.2.3.4' into a single integer ( (1 << 24 | 2 << 16 | 3 << 8 | 4) == 16909060), and then '1.2.0.0' as an integer (16908288) and then the mask as the number of 1 bits from the msb as an integer (16 bits == 4294901760) and then apply the mask with bitwise and to the original address ((16909060 & 4294901760) == 16908288) if the masked address == the network address, then it's in the network.

Answer 2

Pseudocode, building on dustin's answer:

addr_one = ip_addr_to_integer(ip);
addr_two = ip_addr_to_integer(network);
mask = ((1 << (32-slash)) - 1) ^ 0xFFFFFFFF;
return (((addr_one ^ addr_two) & mask) == 0);

If you know that the IP address strings are valid, you can probably use string.split('.') to get the four octets and easily convert them to an integer.