Figure 3 13: Ethernet and Token Ring frame formats in Software

Make QR Code in Software Figure 3 13: Ethernet and Token Ring frame formats

Figure 3 13: Ethernet and Token Ring frame formats
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When an Ethernet or Token Ring station has data to transmit, it encodes the destination address and source address fields with 48-bit numbers that identify the layer 2 locations on the network to receive the frame and the layer 2 device that is transmitting the frame In comparison, at the
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network layer, IP uses a 32-bit address that has no relation to the MAC or layer 2 address Thus, a common problem associated with the routing of an IP datagram to a particular workstation on a LAN involves the delivery of the datagram to its correct destination This delivery process requires an IP device that must transmit a packet via a layer 2 delivery service to obtain the correct MAC or layer 2 address so it can take a packet and convert it into a frame for delivery In the opposite direction, a workstation must be able to convert a MAC address into an IP address Both of these address translation problems are handled by protocols developed to provide an address resolution capability One protocol, known as the Address Resolution Protocol (ARP), translates an IP address into a hardware address The Reverse Address Resolution Protocol (RARP), as its name implies, performs a reverse translation or mapping, converting a hardware layer 2 address into an IP address
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Figure 3 14 illustrates the format of an ARP packet Note that the numbers in some fields indicate the number of bytes when a field spans a four-byte boundary
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Figure 3 14: The ARP packet format
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The 16-bit Hardware Type field indicates the type of network adapter A value of one, for example, indicates a 10 Mbps Ethernet, six indicates an IEEE 802 network, and 16 indicates Asynchronous Transmission Mode (ATM) Similarly, the 16-bit Protocol Type field indicates the protocol for which an address resolution process is being performed For IP, the Protocol Type field has a value of hex 0800 The Hardware Length field denotes the number of bytes in the hardware address The value of this field is six for both Ethernet and Token Ring The Protocol Length field is similar to the Hardware length field, indicating the length of the address for the protocol to be resolved For IPv4, the value of this field is four The Operation field has a value of one for an ARP Request and when a target station responds, the value of this field is changed to two to denote an ARP replay The Sender Hardware Address field is six bytes in length and indicates the hardware address of the station generating the ARP Request or ARP Reply That address is followed by a four-byte Sender IP Address field This field, as its name implies, indicates the IP address of the originator of the packet The next to last field, Target Hardware Address, is originally set to zero in an ARP Request The last field, Target IP Address, is set to the IP address for which the originator needs a hardware address
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To obtain a general appreciation for the operation of ARP, let's say a user located on an Ethernet network wants to transmit a datagram to another computer on the same network The first computer would transmit an ARP packet that would be carried as an Ethernet broadcast frame to all stations on the network Thus, the packet would be transported to all devices on the Ethernet LAN The packet would contain the destination IP address that is known, because the computer is transmitting the IP address to a known location The field in the ARP packet used for the target hardware address, however, is set to all zeros because the transmitting station does not know the destination hardware address Each device on the Ethernet LAN will read the ARP packet as it is transmitted as a broadcast frame, but only the station that recognizes that it has the destination field's IP address will copy the frame off the network and respond to the ARP request When it does, it transmits an ARP reply in which its physical address is inserted in the ARP target hardware address field that was previously set to zero To alleviate the necessity to constantly transmit ARP packets as well as to lower the utilization level of the LAN, the originator records received information in a table known as an ARP cache The use of the ARP cache allows subsequent datagrams with previously learned correspondences between IP addresses and MAC addresses to be quickly transmitted to the appropriate hardware address on the network The standard also calls for devices on the network to update their own ARP table with the MAC and IP address pair of the originator of the ARP request Additionally, when a device initializes its network connection during the boot process, it issues a "gratuitous ARP," which is an ARP request for its own IP address This is how a station determines if another device on the network is using its assigned IP address and becomes aware if there is a conflict Finally, there is a feature in the standard called proxy ARP Proxy ARP is a mechanism that allows a device to answer an ARP request on behalf of another device This feature is necessary when the actual intended recipient of the ARP request does not reside on the same physical subnet as the originator of the request Because the ARP request is a layer 2 broadcast, it is blocked at the router interface If, for example, the originating device has a standard Class C subnet mask, the device would assume that any address within that class resides on its own physical network and would expect it to respond to an ARP request If the network has actually been subnetted into two parts, however, some of the hosts would reside on the other side of a router and the ARP request would not be seen on the second subnet The reason the originating host might not have the correct subnet mask could be because of limitations of the operating system or a misconfiguration In this scenario, the router is aware of both subnets and can answer the ARP request on behalf of other devices on the second subnet by supplying its own MAC address The originating device enters the routers MAC address in its ARP cache and correctly sends packets destined for the end host to the router This feature is enabled by default on Cisco routers An additional use of this feature is that with most operating systems if you enter your own IP address as your default gateway address on your host, the host issues an ARP request for every destination, even those it knows to be on remote networks The benefit of this is that if multiple routers are on the subnet or the router IP address is unknown, your ARP request is automatically answered via the proxy ARP feature without having to configure a single, static default gateway As we can see, ARP provides a well-thought out methodology for equating physical hardware addresses to IP logical addresses and allows IP addressing at layer 3 to occur independently from LAN addressing at layer 2
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Now that we have an appreciation for ARP, we will conclude our examination of the IP by turning our attention to ICMP
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