How to check if IP is private?

While designing systems that auto-scale, one of the roadblocks that I hit was being unable to identify the public IP of the machine I was running my code on.

The following code snippet adapted from here is a very good solution. Note however that this may not be foolproof, so use this at your own discretion:

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public static boolean isPrivateIP( String ip )
{
    boolean local = ip.startsWith( "192.168." ) || ip.startsWith( "10." );
    if ( local ) {
        return true;
    } else {
        if ( ip.startsWith( "172." ) ) {
            Integer second = Integer.parseInt( ip.split( "\\." )[1] );
            if ( second >= 16 && second <= 31 ) {
                return true;
            }
        }
    }
    return false;
}

Software defined private networks like Docker's networks, or GCE private networks start with 10. and LAN usually starts with 192.168.. Amazon's internal IPs usually start with 172. followed by a number between 16 and 31 (inclusive). These might be industry standards for IPs defined in some protocol in fineprint, but I’m not the authority. This might be a starting point.

I’ll discuss how to use this information to find your public IP in the next post.