A Martian packet is a term for a specific kind of IP-routed data packet that has some suspicious markers. In most cases, the address space used has not yet been allocated by the Internet Assigned Numbers Authority (IANA).
System administrators and other network professionals are encouraged to mark and observe bogons or Martian packets to look for signs of denial of service attacks or other hacking or network dysfunctionality.
Martian packets get their name because they seem to come from an impossible source. The name may also be related to the term Martian address, which is used to refer to the unroutable addresses often associated with IP spoofing.