Allow / Block List

An allow / block list is a pair of opposing lists that decide which values — a domain, an IP, a bot, a property value — are let into the data and which are filtered out.

The two lists embody opposite philosophies: an allow list sets the default to “reject unless permitted” and works when the correct set is small and known, while a block list sets the default to “accept unless forbidden” and suits filtering a few named cases out of a large stream. The common trap is unintentionally combining the two, or forgetting that a list usually acts only on new data, not on a retroactive review of old data. Another mistake is treating the list as governance itself; an allow / block list is an enforcement tool, not a policy, and without a clear rule behind it, someone reshapes it differently every month.