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

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.