IP.to_string

You're seeing just the function to_string, go back to IP module for more information.
Link to this function

to_string(ip_addr, style \\ :dots)

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Specs

to_string(addr() | nil, :dots | :hyphens) :: String.t()

Converts an ip address to a string.

For some situations, like converting an ip address to a hostname you might want hyphens as delimiters instead, in which case you should pass :hyphens as the :style term.

Also takes, nil, in which case it emits an empty string.

iex> IP.to_string({255, 255, 255, 255})
"255.255.255.255"

iex> IP.to_string({255, 255, 255, 255}, :hyphens)
"255-255-255-255"

iex> IP.to_string({0, 0, 0, 0, 0, 0, 0, 1})
"::1"