ertain IP address to respond with its MAC address. The TCP/IP software operating in the destination device replies with the requested address and the packet can be addressed and passed on to the sender's Data Link layer. In practice, the sending device keeps a record of the MAC addresses of devices it has recently communicated with, SO it does not need to broadcast a request each time. This ARP table or "cache" is looked at first and a broadcast request is only made if the destination IP address is not in the table. In many cases, a computer will be sending the packet to its default gateway and will find the gateway's MAC address from its ARP table. Routing Routing is the mechanism that enables a data packet to find its way to a destination, whether that is a device in the next room or on the other side of the world. A router compares the destination address of each data packet it receives with a table of addresses held in memory - the router table. If it finds a Wireless Network Logical Architecture match in the table, it forwards the packet to the address associated with that table entry, which may be the address of another network or of a "next-hop" router that will pass the packet along towards its final destination. If the router can't find a match, it goes through the table again looking at just the network ID part of the address (extracted using the subnet mask as described above). If a match is found, the packet is sent to the associated address or, if not, the router looks for a default next-hop address and sends the packet there. As a final resort, if no default address is set, the router returns a "Host Unreachable" or "Network Unreachable" message to the sending IP address. When this message is received it usually means that somewhere along the line a router has failed. What happens if, or when, this elegantly simple structure breaks down? Are there packets out there hopping forever around the Internet, passing from router to router and never finding their destination? The IP header includes a control