|  | 11.2 | Network-to-Network Communications
| 11.2.3 | IP key components
In order for devices to communicate, the sending devices need both, the IP addresses and the MAC addresses, of the destination devices. When they try to communicate with devices whose IP addresses they know, they must determine the MAC addresses. The TCP/IP suite has a protocol, called ARP, that can automatically obtain the MAC address. ARP enables a computer to find the MAC address of the computer that is associated with an IP address. Note: The basic unit of data transfer in IP is the IP packet. Packet processing occurs in software, which means that content and format are not hardware dependent. A packet is divided into two major components: the header, which includes source and destination addresses; and the data. Other types of protocols have their own formats. The IP packet is unique to IP. Note: Another major component of IP is Internet Control Message Protocol (ICMP). This protocol is used by a device to report a problem to the sender of a message. For example, if a router receives a packet that it cannot deliver, it sends a message back to the sender of the packet. One of the many features of ICMP is echo-request/echo-reply, which is a component that tests whether a packet can reach a destination by pinging the destination.