IP falls in CIDR range

Question

I have an IP like this: 12.12.12.12
I'm looping through different IP ranges (in 12.12.12.0/24 (example)) format, and trying to see if the IP is in the range.
I have tried various methods such as inet_addr and comparing but I can't seem to get it.
Is there an easy way to do this? I'm using Windows.

Answer 1

Take the binary representation and zero out what is not matching your network mask.

Clarification: Let's say you have the IP a.b.c.d and want to match it to e.f.g.h/i then, you can throw the IP into one unsigned integer, uint32_t ip = a<<24 + b<<16 + c<<8 + d and do the same with uint32_t range = e<<24 + f<<16 + g<<8 + h. Now you can use your network mask: uint32_t mask = (~0u) << (32-i). Now, you can simply check if ip "is in" range by comparing them: ip & mask == range & mask.

Answer 2

Just test whether:

(ip & netmask) == (range & netmask)

You can determine the netmask from the CIDR parameters range/netbits as follows:

uint32_t netmask = ~(~uint32_t(0) >> netbits);