approved by someone

Don't solve problems that don't exist.

This is very much readable if you make it a bit less cramped (like in my solution).

C++ translation, with random tests

Point 1 is now fixed too.

Point 2 is now fixed

lgtm, approved

this piece of code is a friendly reminder that heavy string-work should be done in any language but C or assembly

C++ translation ready for review (author is inactive).

Rejected:

1. you don't test the value of first at all.
2. there's lots of "C++ is just C with classes"-type of code; e. g. using a modifiable pointer as a way to return two values is usually bad practice in modern C++ (std::pair or, for this case specifically, std::optional is usually the better choice here).

C — so minimal and yet sometimes so elegant.

• your do_test is in reverse order; Assert::That's syntax is: Assert::That(actual, Equals(expected))
• why are you using rand()? if you need a good RNG, you can use <random>:

#include <random>

int randint(int min, int max) {
    static std::mt19937 engine(std::random_device{}()); // initialize once to avoid doing it again on every function call
    std::uniform_int_distribution<int> distribution(min, max); // note: the range is [min, max]
    
    return distribution(engine);
}

• using namespace std; should, preferably, be removed; solution setup doesn't compile because your tests use ostringstream without std::.
• i think it would be a good idea to, in case of assertion failure, tell the user which IPs were passed to the function:

void do_test(const std::string& input_start, const std::string& input_end, long expected) {
    Assert::That(
        ips_between(input_start, input_end),
        Equals(expected),
        ExtraMessage(std::string("start: ") + input_start + ", end: " + input_end + "\n")
    );
}

• this one is mostly a nitpick, but you should really be passing strings (and other objects that aren't cheap to copy) by const reference instead of value (which results in a full copy), e.g.:

long ips_between(const std::string& start, const std::string& end) {
  // TODO
  return 0;
}