puter 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. Layer 3 protocols determine whether data passes beyond the network layer to higher levels of the OSI model. A data packet must contain both, a destination MAC address and a destination IP address. If it lacks one or the other, the data will not pass from Layer 3 to the upper layers. In this way, MAC addresses and IP addresses act as checks and balances for each other. After devices determine the IP addresses of the destination devices, they can add the destination MAC addresses to the data packets. There are a variety of ways that devices can determine the MAC addresses they need to add to the encapsulated data. Some keep tables that contain all the MAC addresses and IP addresses of other devices that are connected to the same LAN. They are called Address Resolution Protocol (ARP) tables, and they map IP addresses to the corresponding MAC addresses. ARP tables are sections of RAM memory, in which the cached memory is maintained automatically on each of the devices. It is a rare occasion when you must make an ARP table entry manually. Each computer on a netwo...
