How to determine whether my internet IP belongs to a given IP list using Qt

Question

I have following IP list (in CIDR format) stored in a TXT file:<

58.200.0.0/13
202.115.0.0/16
121.48.0.0/15
219.224.128.0/18
...

But I don't know how can I determine whether my IP belongs to this list. I use the Qt C++ framework on Windows platform.

Answer 1

First you need to break up each CIDR notation range into a network (the dotted IP address) part, and a number of bits. Use this number of bits to generate the mask. Then, you only need to test if (range & mask) == (your_ip & mask), just as your operating system does:

Some psuedo-C code:

my_ip = inet_addr( my_ip_str )            // Convert your IP string to uint32
range = inet_addr( CIDR.split('/')[0] )   // Convert IP part of CIDR to uint32

num_bits = atoi( CIDR.split('/')[1] )     // Convert bits part of CIDR to int
mask = (1 << num_bits) - 1                // Calc mask

if (my_ip & mask) == (range & mask)
    // in range.

You can probably find a library to help you with this. Boost appears to have an IP4 class which has < and > operators. But you'll still need to do work with the CIDR notation.

Ref:

• IP falls in CIDR range

Answer 2

Walking through the Qt Documentation, I came across QHostAddress::parseSubnet(const QString & subnet), which can take a CIDR-style IP range and is new in Qt 4.5. Thus I could write the following code to solve it: (assume myIP is of type QHostAddress)

if(myIP.isInSubnet(QHostAddress::parseSubnet("219.224.128.0/18")) {
    //then the IP belongs to the CIDR IP range 219.224.128.0/18
}

As for better understanding and insights into the problem, @Jonathon Reinhart 's answer is really helpful.