When a computer wants to send data within a network (not across i.e. via a router), it usually makes use of the NIC MAC address to send such data. Every computer keeps a record of IP-to-MAC-address mapping. This is called the ARP cache. It is through the help of the ARP cache that the computer knows which NIC to send data meant for a particular IP address. A computer populates (stores an entry) its ARP cache when it receives an arp request.

In networking, a packet is the basic unit of data. Every data sent across the network is usually done as a packet. This means that if an adversary can gain access to a packet, such an adversary can launch sophisticated attacks.