@Override
  public void start() throws NatInitializationException {
    if (started.compareAndSet(false, true)) {
      doStart();
    } else {
      LOG.warn("Attempt to start an already-started {}", getClass().getSimpleName());
    }
  }
Retrieve external ip address completable future.

@return the completable future
nat/src/main/java/org/hyperledger/besu/nat/core/AbstractNatManager.java