GeezerVison

See the linter issue, capitalize the comment

Not that your fix is wrong but the mask calculation has undefined behavior (signed out-of-range shift) for prefix > 30 so a more comprehensive fix is this:

There's another instance around line 296.

prefix arguably shouldn't be an int but e.g. a uint8_t because what does a less-than-zero prefix even mean? That's a bigger change though.

While I agree with your observation, currently there is a check around prefixes < 0 or > 32:

$ ~/node/bin/node 
> const bl = new net.BlockList()
undefined
> bl.addSubnet('1.1.0.0', -1, 'ipv4')
Uncaught:
RangeError [ERR_OUT_OF_RANGE]: The value of "prefix" is out of range. It must be >= 0 && <= 32. Received -1
    at __node_internal_captureLargerStackTrace (node:internal/errors:477:5)
    at new NodeError (node:internal/errors:388:5)
    at __node_internal_ (node:internal/validators:92:13)
    at BlockList.addSubnet (node:internal/blocklist:107:9) {
  code: 'ERR_OUT_OF_RANGE'
}

If you still feel that this is required, please let me know.

That's right, and there are also a couple of additional C++ checks that enforce the range but still, switching to uint8_t would help code clarity. That's just a suggestion though, not a hard requirement for this PR to land.

The UB should be fixed though. It's dangerous.

Apologies for the delay on this. What is the recommended approach to add the UB fix on top, should be a separate commit or should the previous commit be amended with the fix?

Doesn't matter, what you prefer. It gets squashed into a single commit before merge anyway.