IP address
An IP address serves two principal functions. It identifies the host, or more specifically its network interface, and it provides the location of the host in the network, and thus the capability of establishing a path to that host. Its role has been characterized as follows: "A name indicates what we seek. An address indicates where it is. A route indicates how to get there."
The header of each IP packet contains the IP address of the sending host, and that of the destination host.
Martian packet
A Martian packet is an IP packet seen on the public internet that contains a source or destination address that is reserved for special-use by Internet Assigned Numbers Authority (IANA). On the public Internet, such a packet’s source address is either spoofed, and it cannot actually originate as claimed, or the packet cannot be delivered.