drupal-entity-access-ip-address.php

<?php

/**
 * Implements hook_entity_access().
 */
function HOOK_entity_access(EntityInterface $entity, $operation, AccountInterface $account) {

  // Only allow access to the test page.
  $testPageNid = 111111;
  if ((int) $entity->id() !== $testPageNid) {
    return AccessResult::neutral();
  }

  $allowlist = [
    // Diffy.
    '3.216.56.216',

    // Me.
    '22.222.22.222',
  ];

  $requestIP =  \Drupal::request()->getClientIp();
  $isOnAllowlist = in_array($requestIP, $allowlist, TRUE);

  return AccessResult::allowedIf($isOnAllowlist);

}