Send binary data with cfsocketsenddata

Question

I try to send a UDP package for Wake on Lan. The first part of the package needs to be 6*FF in hexadecimal code followed by 16 * mac address in hex. How can I send the NSString in hex value? As now it sends the value 255 as 32 35 35 instead of FF.

I have the following code:

        NSMutableString *messageData= [NSMutableString new];
        [messageData appendString: @"255255255255255255"];
        for (int i = 0; i <= 16; i++) {
            [messageData appendString:(@"%@ ",self.macAddress)];
        }

        char const *messageDataChar = [messageData UTF8String];
        CFDataRef Data = CFDataCreate(NULL, (const UInt8*)messageDataChar, strlen(messageDataChar));

        NSData *destAddrData = [NSData dataWithBytes:&addr length:sizeof(addr)];

        CFSocketError errorDetails = CFSocketSendData(socket, (__bridge CFDataRef) destAddrData, Data, 0);

Todo: inplement a broadcast instead of sending to an ip address.

Wireshark shows the following package data being sent when trying with mac address aa:bb:cc:dd:ee:ff :

0000  32 35 35 32 35 35 32 35 35 32 35 35 32 35 35 32   2552552552552552
0010  35 35 61 61 62 62 63 63 64 64 65 65 66 66 61 61   55aabbccddeeffaa
0020  62 62 63 63 64 64 65 65 66 66 61 61 62 62 63 63   bbccddeeffaabbcc
0030  64 64 65 65 66 66 61 61 62 62 63 63 64 64 65 65   ddeeffaabbccddee
0040  66 66 61 61 62 62 63 63 64 64 65 65 66 66 61 61   ffaabbccddeeffaa
0050  62 62 63 63 64 64 65 65 66 66 61 61 62 62 63 63   bbccddeeffaabbcc
0060  64 64 65 65 66 66 61 61 62 62 63 63 64 64 65 65   ddeeffaabbccddee
0070  66 66 61 61 62 62 63 63 64 64 65 65 66 66 61 61   ffaabbccddeeffaa
0080  62 62 63 63 64 64 65 65 66 66 61 61 62 62 63 63   bbccddeeffaabbcc
0090  64 64 65 65 66 66 61 61 62 62 63 63 64 64 65 65   ddeeffaabbccddee
00a0  66 66 61 61 62 62 63 63 64 64 65 65 66 66 61 61   ffaabbccddeeffaa
00b0  62 62 63 63 64 64 65 65 66 66 61 61 62 62 63 63   bbccddeeffaabbcc

What I need is the following:

0000  ff ff ff ff ff ff aa bb cc dd ee ff aa bb cc dd   ................
0010  ee ff aa bb cc dd ee ff aa bb cc dd ee ff aa bb   ................

I can't really find how to work with hex values in objective C.