What is the first part of an IP address?
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."[2] The header of each IP packet contains the IP address of the sending host, and that of the destination host.
Network interface layer security
At the data link layer a logical IP address assigned by the network layer is translated into a physical MAC address. In order to ensure reliable data communications all switches in the network must maintain up-to-date tables for mapping logical (IP) to physical (MAC) addresses. If a client or switch is unsure of the IP-to-MAC mapping of a data packet it receives it will send an Address Resolution Protocol (ARP) message to the nearest switch asking for the MAC address associated with the particular IP address. Once this is accomplished the client or switch will update its table to reflect the new mapping. In an ARP spoofing attack the adversary broadcasts the IP address of the machine to be attacked along with its own MAC address. All neighboring switches will then update their mapping tables and begin transmitting data destined to the attacked system’s IP address to the attacker’s MAC address. Such an attack is commonly referred to as a "man in the middle" attack.