Improve CIDR validation

[johngmyers]

Would have caught the user error in #8040

[k8s-ci-robot]

Hi @johngmyers. Thanks for your PR.

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[zetaab]

/ok-to-test

[johngmyers]

/retest

[johngmyers]

/retest

[fgbreel]

Thank you! 👍

[justinsb]

Thanks for fixing this @johngmyers - and our tests even made the same mistake!

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: johngmyers, justinsb

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