The Cymru bogons blacklist is a Domain Name System (DNS)-based blacklist based on IP address ranges defined in Request for Comment (RFC) 1918 and RFC 5735. Bogons are legitimate IP addresses that have not yet been allocated or delegated by the Internet Assigned Numbers Authority (IANA) or a delegated Regional Internet Registry (RIR). This includes IPv4 and IPv6 bogons. Mailbox providers typically block bogons because they are assumed to be caused by accidental or malicious misconfiguration.
- Type of list: This is a public blacklist.
- Listings occur: At the IP level.
- Criteria for getting listed: The Cymru bogons blacklist has no listing criteria. The address space is well known, either being RFC-defined private IP address space or unused and unallocated IP address space.
- Impact on deliverability: Minimal; legitimate senders will never see a listing at this blacklist.
- Regional impact: Unknown, not used by many mailbox providers.
- Getting removed from this blacklist: Removal from the bogons list happens when the IPv4 or IPv6 address space, which is designated as not in use, is allocated by IANA and assigned to a RIR, which in turn allocates it to a mailbox provider or end user.