https://pwnbox.io/challenges/baby-ssrf
the goal of this lab is to tell you about other forms that represent the same thing.
in this lab if you just go there and put 127.0.0.1 or localhost you would get block, because there are internals. but if the block list is not good enough it would not block other forms of localhost like this ones:
http://0/
http://127.1
http://127.0.1
and done just request to one of this and the given port (9999) and you got the flag.