_isIPv4 (addr) {
    const IPv4Pattern = /^((?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])[.]){3}(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])$/
    return IPv4Pattern.test(addr)
  }
Return `true` if string is a valid IPv4 address.
@param {string} addr
@return {boolean}