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 fragmentation attack
The source system sets "Identification" field in each packet to a unique value for all packets which use the same source IP address, destination IP address, and "Protocol" values, for the lifetime of the packet on the internet. This way the destination can distinguish which incoming fragments belong to a unique packet and buffer all of them until the last fragment is received. The last fragment sets the "More Fragment" bit to 0 and this tells the receiving station to start reassembling the data if all fragments have been received.