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.
IP traceback
Belenky and Ansari, outline a deterministic packet marking scheme. They describe a more realistic topology for the Internet – that is composed of LANs and ASs with a connective boundary – and attempt to put a single mark on inbound packets at the point of network ingress. Their idea is to put, with random probability of .5, the upper or lower half of the IP address of the ingress interface into the fragment id field of the packet, and then set a reserve bit indicating which portion of the address is contained in the fragment field. By using this approach they claim to be able to obtain 0 false positives with .99 probability after only 7 packets.