const string validIPv6Chars = "0123456789abcdefABCDEF";
    
    bool isValidIPv4Block(string& block) {
    	int num = 0;
    	if (block.size() > 0 && block.size() <= 3) {
    	    for (int i = 0; i < block.size(); i++) {
    	        char c = block[i];
    	        // special case: if c is a leading zero and there are characters left
    	        if (!isalnum(c) || (i == 0 && c == '0' && block.size() > 1))
    		    return false;
    	        else {
    		    num *= 10;
    		    num += c - '0';
    	        }
    	    }
    	    return num <= 255;
    	}
    	return false;
    }
    
    bool isValidIPv6Block(string& block) {
    	if (block.size() > 0 && block.size() <= 4) {
    	    for (int i = 0; i < block.size(); i++) {
    	        char c = block[i];
    	        if (validIPv6Chars.find(c) == string::npos)
    	    	    return false;
    	    }
    	    return true;
    	}
    	return false;
    }
    
    string validIPAddress(string IP) {
    	string ans[3] = {"IPv4", "IPv6", "Neither"};
    	stringstream ss(IP);
    	string block;
    	// ipv4 candidate
    	if (IP.substr(0, 4).find('.') != string::npos) {
    	    for (int i = 0; i < 4; i++) {
    		if (!getline(ss, block, '.') || !isValidIPv4Block(block))
    	   	    return ans[2];
    	    }
    	    return ss.eof() ? ans[0] : ans[2];
    	}
    	// ipv6 candidate
    	else if (IP.substr(0, 5).find(':') != string::npos) {
    	    for (int i = 0; i < 8; i++) {
    		if (!getline(ss, block, ':') || !isValidIPv6Block(block))
    		    return ans[2];
    	    }
    	    return ss.eof() ? ans[1] : ans[2];
    	}
    
    	return ans[2];
    }