Curs PC si internet cap 11.3 Advanced ARP Commands

11.3 Advanced ARP Commands 11.3.1 Default gateway Instructor Note The concept of default gateway is introduced to continue the detailed description of the functioning of a router. You might have the students discover the default gateway IP address for their own machines by looking at the TCP/IP properties tab in Windows. In order for a device to communicate with another device on another network, you must supply it with a default gateway. A default gateway is the IP address of the interface on the router that connects to the network segment that the source host is located on. The default gateways IP address must be in the same network segment as the source host. If no default gateway is defined, communication is possible only on the devices own logical network segment. The computer that sends the data does a comparison between the IP address of the destination and its own ARP table. If it finds no match, it must have a default IP address to use. Without a default gateway, the source computer has no destination MAC address, and the message is undeliverable. 11.3 Advanced ARP Concepts 11.3.2 Problems with sending data to nodes on different subnets Instructor Note The purpose of this target indicator is highlight two general problems of internetworking. For both delivery and handling after delivery, hosts on different subnetworks must have protocols that have features beyond the LAN protocols discussed earlier in the course. One of the major problems in networking is how to communicate with devices that are not on the same physical network segment. There are two parts to the problem. The first is obtaining the MAC address of the destination host, and the second is transferring the data packets from one network segment to another, to get to the destination host. 11.3 Advanced ARP Concepts 11.3.3 How ARP sends data to remote networks Instructor Note This target indicator has many aspects to it. First, there is the notion that ARP uses broadcast packets in order to find a destination MAC address. Review with the students what an ARP broadcast is. But routers do not forward broadcast packets and thus a destination host on another subnetwork will not receive the ARP broadcast. This is actually a desirable property of routers; they create separate, smaller broadcast domains. If they didn't, the different networks attached to the router would become flooded with each other's broadcasts. So the host cannot rely on ARP to directly get information about other networks and hosts.Instead the host relies on the router interface which is the default gateway for the host. The default gateway router will reply to the host's ARP. When the packet is delivered to the router, it then uses its routing tables to determine which network and hence which interface to which the packet will be delivered. ARP uses broadcast packets to accomplish its function. Routers, however, do not forward broadcast packets. In order for a device to send data to the address of a device that is on another network segment, the source device sends the data to a default gateway. The default gateway is the IP address of the router interface that is connected to the same physical network segment as the source host. The source host compares the destination IP address and its own IP address to determine if the two IP addresses are located on the same segment. If the receiving host is not on the same segment, the source host sends the data to the default gateway. 11.3 Advanced ARP Concepts 11.3.4 Proxy ARP Instructor Note The purpose of this target indicator is to introduce some more vocabulary to the students. Proxy ARP is another important protocol with which they should be familiar. Proxy ARP is a variation of the ARP protocol in which an intermediate device (e.g. router) sends an ARP response, on behalf of an end node, to the requesting host. Routers running proxy ARP capture ARP packets and respond with their MAC addresses for those requests in which the IP address is not in the range of addresses of the local subnet. In the previous description of how data is sent to a host on a different subnet, the default gateway is configured. If the source host does not have a default gateway configured, it sends an ARP request. All hosts on the segment, including the router, receive the ARP request. The router compares the IP destination address with the IP subnet address to determine if the destination IP address is on the same subnet as the source host. If the subnet address is the same, the router discards the packet. The reason that the packet is discarded is that the destination IP address is on the same segment as the source's IP address, and another device on the segment will respond to the ARP request. The exception to this is that the destination IP address is not currently assigned, which will generate an error response on the source host. If the subnet address is different, the router will respond with its own MAC address for the interface that is directly connected to the segment on which the source host is located. This is the proxy ARP. Since the MAC address is unavailable for the destination host, the router supplies its MAC address in order to get the packet so that it can forward the ARP request (based on the destination IP address) to the proper subnet for delivery. 11.3 Advanced ARP Concepts 11.3.5 Four Layer 3 flowcharts Instructor Note As a review of the concepts learned, the students should be asked to create flowcharts for ARP, RARP, BOOTP, and DHCP. The flowchart for ARP is given as an example. Flowcharting was introduced in Chapter 1, and should be reviewed periodically. It is a concise way to express the complex networking processes the students are learning. Create flowcharts for the following processes: ARP RARP BOOTP DHCP