@Override
  public String toString() {
    return String.format(
        "[%s - %s] %s:%d ==> %s:%d",
        natServiceType, protocol, internalHost, internalPort, remoteHost, externalPort);
  }
Gets internal port.

@return the internal port
nat/src/main/java/org/hyperledger/besu/nat/core/domain/NatPortMapping.java