Function fallback_dc_addr 

pub fn fallback_dc_addr(dc_id: i32) -> &'static str

Return the statically known IPv4 address for a Telegram DC.

Used as a fallback when the DC is not yet in the session’s dc_options table (i.e. first migration to a DC we haven’t talked to before).

Source: https://core.telegram.org/mtproto/DC