IPAddress address(100, 20, 3, 255);

String toString(IPAddress& ip) {
  String ips;
  ips.reserve(16);
  ips = ip[0];  ips += ':';
  ips += ip[1]; ips += ':';
  ips += ip[2]; ips += ':';
  ips += ip[3];
  return ips;
}

void setup() {
  Serial.begin(115200);
  String message = "Your IP address is ";
  message += toString(address);
  Serial.println(message);
}

void loop() {}