workers.js

addEventListener('fetch', event => {
  event.respondWith(fetchAndApply(event.request))
})

async function fetchAndApply(request) {  
  if (request.headers.get('cf-connecting-ip') !== '225.0.0.1') {
    return new Response('Sorry, this page is not available.',
        { status: 403, statusText: 'Forbidden' })
  }

  return fetch(request)
}