Here is an example, given IP range specified as 157.166.224.26/10, check if 157.166.21.21 is within this range. It seems that for .NET and C++/C folks the CIDR and IP range check come with the standard libraries. This is not the case for java, as far as I know. So, here is how I’ve done it in Java, after looking at java source code for IntetAddress, and a similar C implementation.

Let’s see what the range 157.166.224.26/10 really specifies. Here is a handy calculator which I used when playing with CIDR. So, as we can see, 157.166.224.26/10 really means you have to accept IPs from 157.128.0.0 to 157.191.255.255. The range contains 2^(32-10) IP addresses.

// Step 1. Convert IPs into ints (32 bits). 
// E.g. 157.166.224.26 becomes 10011101  10100110  11100000 00011010
int addr = (( 157 << 24 ) & 0xFF000000) 
           | (( 166 << 16 ) & 0xFF0000) 
           | (( 224 << 8 ) & 0xFF00) 
           |  ( 26 & 0xFF);

// Step 2. Get CIDR mask
int mask = (-1) << (32 - 10);

// Step 3. Find lowest IP address
int lowest = addr & mask;

// Step 4. Find highest IP address
int highest = lowest + (~mask);

Now, your IP is in the range if lowest <= IP && IP <= highest.

Hopefully, no bugs here.