IP.Subnet.config_from_string

You're seeing just the function config_from_string, go back to IP.Subnet module for more information.
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config_from_string(config_str)

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finds an ip address and subnet together from a config representation (this is an ip/cidr string where the ip is not necessarily the routing prefix for the cidr block).

returns {:ok, ip, subnet} if the config string is valid; {:error, reason} otherwise.