code: static public Key make(byte[] kb, byte rf, byte systemType, H2ONode... replicas) {
      // no more than 3 replicas allowed to be stored in the key
      assert 0 <=replicas.length && replicas.length<=3;
      assert systemType<32; // only system keys allowed
      // Key byte layout is:
      // 0 - systemType, from 0-31
      // 1 - replica-count, plus up to 3 bits for ip4 vs ip6
      // 2-n - zero, one, two or 3 IP4 (4+2 bytes) or IP6 (16+2 bytes) addresses
      // 2-5- 4 bytes of chunk#, or -1 for masters
      // n+ - repeat of the original kb
      AutoBuffer ab = new AutoBuffer();
      ab.put1(systemType).put1(replicas.length);
      for( H2ONode h2o : replicas )
        h2o.write(ab);
      ab.put4(-1);
      ab.putA1(kb,kb.length);
      return make(Arrays.copyOf(ab.buf(),ab.position()),rf);
    }
docs: Make a Key which is homed to specific nodes.