How exactly does is-reachable work?

[imthenachoman]

We use clasp and clasp uses is-reachable. clasp has started failing on our corporate network because is-reachable is returning false for specific domains.

I don't know TypeScript or how any of this code works.

I am trying to understand exactly how is-reachable works -- how does it determine if a host is reachable or not -- so I can have our IT guys fix it on our end.