These are the tools for the plumbing between a request and a response: the address it goes to, the URL it carries, the status and content type that come back, and the permissions on the file that serves it. None of them makes a network call, which is the point. A subnet, a URL or a status code can be worked out from the text alone, so nothing about your infrastructure leaves the tab.
The CIDR calculator turns an IPv4 prefix such as 10.0.0.0/22 into its network and broadcast addresses and its host range, and counts the usable hosts: two fewer than the block size for every prefix up to /30, with the /31 point-to-point exception that RFC 3021 allows. The IP address converter writes one address every way it can be written — dotted, decimal, hexadecimal, binary and the IPv6 forms. That matters more than it sounds: 2130706433 and 0177.0.0.1 both reach 127.0.0.1, which is why filters that compare addresses as strings get bypassed.
On the HTTP side, HTTP status codes lists every registered code plus the ones servers and CDNs invented, with what each means for a client deciding whether to try again. A 429 or a 503 usually deserves a retry after the Retry-After delay; a 400 or a 404 will fail the same way however often it is sent. The MIME type lookup maps file extensions to media types and back and gives the Content-Type header to send. A wrong type is not cosmetic: with X-Content-Type-Options: nosniff set, a browser refuses to run a script or apply a stylesheet served under the wrong type.
The URL parser splits a URL into scheme, host, port, path, query and fragment and decodes each part, which settles arguments about where a redirect or a tracking link really points. The chmod calculator translates between octal permissions such as 755 and symbolic ones such as rwxr-xr-x, and gives the command to apply them. The classic mistake is chmod 777 to make an error go away: it lets every account on the machine rewrite the file, and the real fault is almost always ownership, which chown fixes.