Question

I want to check if an IP address is in a certain range, matching by "*" only. For example, “202.121.189.8” is in “202.121.189.*“.

The scenario is that I have a list of banned IPs, some of them contains "*", so I wrote a function, it works fine so far:

static bool IsInRange(string ip, List<string> ipList)
{
    if (ipList.Contains(ip))
    {
        return true;
    }

    var ipSets = ip.Split('.');
    foreach (var item in ipList)
    {
        var itemSets = item.Split('.');
        for (int i = 0; i < 4; i++)
        {
            if (itemSets[i] == "*")
            {
                bool isMatch = true;
                for (int j = 0; j < i; j++)
                {
                    if (ipSets[i - j - 1] != itemSets[i - j - 1])
                    {
                        isMatch = false;
                    }
                }
                if (isMatch)
                {
                    return true;
                }
            }
        }
    }
    return false;
}

Test code:

string ip = "202.121.189.8";
List<string> ipList = new List<string>() { "202.121.168.25", "202.121.189.*" };

Console.WriteLine(IsInRange(ip, ipList));

But I think what i wrote is very stupid, and I want to optimize it, does anyone have an idea how to simplify this function? not to use so many “for….if…”.

Answer 1

A good idea would be to represent the banned subnets in a form of a pair: mask + base address. So your check will look like that:

banned = (ip & mask == baseaddress & mask);

For 11.22.33.* the base address will be 11*0x1000000 + 22*0x10000 + 33*0x100, mask will be 0xffffff00.

For single address 55.44.33.22 the address will be 55*0x1000000 + 44*0x10000 * 33*0x100 + 22, mask will be 0xffffffff.

You’ll need to convert the address to a 32-bit int as a separate procedure.

After that all, your code will look like that:

int numip = ip2int(ip);
bool isIpBanned = banList.Any(item =>
            numip & item.mask == item.baseaddress & item.mask);

By the way, this way you’ll be able to represent even bans on smaller subsets.

int ip2int(string ip) // error checking omitted
{
    var parts = ip.Split('.');
    int result = 0;
    foreach (var p in parts)
        result = result * 0x100 + int.Parse(p);
}


class BanItem { public int baseaddres; public int mask; }

BanItem ip2banItem(string ip)
{
    BanItem bi = new BanItem() { baseaddres = 0, mask = 0 };
    var parts = ip.Split('.');
    foreach (var p in parts)
    {
        bi.baseaddress *= 0x100;
        bi.mask *= 0x100;
        if (p != "*")
        {
            bi.mask += 0xff;
            bi.baseaddress += int.Parse(p);
        }
    }
    return bi;
}

banList = banIps.Select(ip2banItem).ToList();