h data packet and is used by routing software in the network or Internet to establish the source and destination of each packet. While IP addresses, which are defined at the Network layer, link the billions of devices connected to the Internet into a single virtual network, the actual transmission of data frames between devices relies on the MAC addresses of the network interface cards (NICs), rather than the logical IP addresses of each NIC's host device. Translation between the Layer 3 IP address and the Layer 2 MAC address is achieved using Address Resolution Protocol (ARP), which is described in the Section "Address Resolution Protocol, p. 16". IP Addressing The 32-bit IP address is usually presented in "dot decimal" format as a series of four decimal numbers between 0 and 255, for example; 200.100.50.10. This could be expanded in full binary format as 11001000.01100100.00110010.00001010 As well as identifying a computer or other networked device, the IP address also uniquely identifies the network that the device is connected to. These two parts of the IP address are known as the host ID and the network ID. The network ID is important because it allows a device Chapter Two transmitting a data packet to know what the first port of call needs to be in the route to the packet's destination. If a device determines that the network ID of the packet's destination is the same as its own network ID, then the packet does not need to be externally routed, for example through the network's gateway and out onto the Internet. The destination device is on its own network and is said to be "local" (Table 2-2). On the other hand, if the destination network ID is different from its own, the destination is a remote IP address and the packet will need to be routed onto the Internet or via some other network bridge to reach its destination. The first stage in this will be to address the packet to the network's gateway. This process uses two more 32-bit numbers, the "subnet mask" and the "default gateway". A device determines th