3. Original packet <src=Orig, dst=CoA(MT)> <src=Orig, dst=HAddr(MT)> Foreign network network network Destination mobile terminal (MT), 1. Original packet Source HAddr <src=Orig, dst=HAddr(MT)> HA=Home agent FA=Foreign agent Figure 6.43 MIP architecture and message flow. The advantage of MIP is that it relies on ordinary IP routing and the IP used by both source and destination needs no modifications. However, it introduces additional delay that is caused by tunneling of packets via HA. 6.6.5 Address Resolution Protocol When a router is connected to other routers by point-to-point links, and it receives a packet to be routed, it just has to find out to which port it forwards the IP packet. However, usually a router connecting a corporate network to the Internet is connected to a LAN to which a set of hosts and other routers are also connected. To transmit an IP packet via a LAN to a certain host, the MAC address of the destination host must be known. Configuration files updated by a network manager could define a connection between MAC and IP addresses but this would be a permanent headache in large networks. The Address Resolution Protocol maps an IP address to a MAC address as illustrated in Figure 6.44. A router of class C network 221.109.65.0 is connected to the external Internet and the network is divided into subnet- works 1 and 2 with subnet mask 255.255.255.192. When the packet from external network arrives (1), the router uses a subnet mask to find out the subnetwork of the destination host. In our example in Figure 6.44, the sub- net mask gives 221.109.65.64 (2), which is subnet 1 connected to port 1. If the router does not know the MAC address of the destination, it sends an ARP request (3) carrying its own IP and MAC addresses in addition to the destination IP address. Broadcast MAC address (111...11) is used as a desti- nation hardware address and the type field in the Ethernet frame is set to hex 0805 (2,054 decimal) to indicate that this is an ARP frame. All hosts in sub- network 1 receive the frame