Arduino ethernet client reading commands from arduino ethernet server

I have an outdoors Arduino ethernet client that's connected via ethernet to an indoors Arduino ethernet server. The client requests a series of settings from the server every 30 seconds, and the server sends the settings as a string of 1's and 0's. Currently there are 6 settings to be read, but I might add more in the future.

Could you please examine my client code and tell me if you see any problems? So far it seems to be working, but I'm worried that maybe I'm not doing things optimally, or maybe I'm overlooking some problems in the code.

As a secondary question, I'm wondering if you think the code would be faster and more stable (and worth the extra complexity) if instead of sending 1's and 0's as strings, I somehow made it send hex values that could be interpreted as bit field settings.

This is the client code:

EthernetClient client;
const char server[] = "192.168.0.106";
byte brightness = 0, failedCounter = 0, xbeeEnable = 0;
unsigned long lastConnectionTime = 0; // last time you connected to the server
boolean lastConnected = false; // state of the connection last time through the main loop
const word postingInterval = 30 * 1000; // delay between updates
String readString;

void setup() {
  Serial.begin(9600);
  startEthernet();
  delay(3000);
}

void loop() {
  if (client.available()) {
    char c = client.read();
    if (readString.length() < 7) readString += c;
  }

  if (!client.connected() && lastConnected) client.stop();

  if (!client.connected() && (millis() - lastConnectionTime > postingInterval)) {
    httpRequest();
    if (readString.length() > 2) { //check in case it didn't receive the entire string, because sometimes the string was empty

//below are 2 examples of reading settings. I removed the others for cleaner code    
      if (readString[4] == '1') brightness = 255; else brightness = 10;
      if (readString[5] == '1') xbeeEnable = 1; else xbeeEnable = 0;

      readString = "";
    }
  }
  lastConnected = client.connected();
  if (failedCounter > 2 ) startEthernet();
}

void startEthernet()
{
  client.stop();
  delay(3000);
  failedCounter = 0;
  Ethernet.begin(mac);
  delay(5000);
}

And this is what the server is sending (an example output would be: "100011"):

client.print(redMailOn);
client.print(whiteMailOn);
client.print(blueMailOn);
client.print(yellowMailOn);
client.print(mailNotifierBrightness);
client.println(enableXbeeTransmitter);
delay(1); client.stop();