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9.3 | |
Obtaining an IP address | |
| 9.3.7 | |
Address Resolution Protocol (ARP) |
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With TCP/IP networking, a data packet must
contain both a destination MAC address and a destination IP address.
If the packet is missing either one, the data will not pass from Layer
3 to the upper layers. In this way, MAC addresses and IP addresses act
as checks and balances for each other. After devices determine the IP
addresses of the destination devices, they can add the destination MAC
addresses to the data packets.
Some devices will keep tables that
contain MAC addresses and IP addresses of other devices that are
connected to the same LAN.
When a source determines the IP address for a destination, it then consults the ARP table in order to locate the MAC address for the destination. If the source locates an entry in its table, destination IP address to destination MAC address, it will associate the IP address to the MAC address and then uses it to encapsulate the data. The data packet is then sent out over the networking media to be picked up by the destination device. There are two ways that devices can
gather MAC addresses that they need to add to the encapsulated data.
The computer that requires an IP and MAC address pair broadcasts an ARP request. All the other devices on the local area network analyze this request. If one of the local devices matches the IP address of the request, it sends back an ARP reply that contains its IP-MAC pair. If the IP address is for the local area network and the computer does not exist or is turned off, there is no response to the ARP request. In this situation, the source device reports an error. If the request is for a different IP network, there is another process that can be used. Routers do not forward broadcast
packets. If the feature is turned on, a router performs a proxy ARP.
Another method to send data to the
address of a device that is on another network segment is to set up a
default gateway.
If the default gateway on the host or the proxy ARP feature on the router is not configured, no traffic can leave the local area network. One or the other is required to have a connection outside of the local area network.
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These are called Address Resolution Protocol (ARP) tables.
ARP tables are stored in RAM memory, where the cached information is
maintained automatically on each of the devices. It is very unusual
for a user to have to make an ARP table entry manually. Each device on
a network maintains its own ARP table. When a network device wants to
send data across the network, it uses information provided by the ARP
table. 
One way is
to monitor the traffic that occurs on the local network segment. All
stations on an Ethernet network will analyze all traffic to determine
if the data is for them. Part of this process is to record the source
IP and MAC address of the datagram to an ARP table. So as data is
transmitted on the network, the address pairs populate the ARP table.
Another way to get an address pair for data transmission is to
broadcast an ARP request.
Proxy ARP
is a variation of the ARP protocol. In this variation, a router sends
an ARP response with the MAC address of the interface on which the
request was received, to the requesting host. The
router responds with the MAC addresses for those requests in which the
IP address is not in the range of addresses of the local subnet.
The
default gateway is a host option where the IP address of the router
interface is stored in the network configuration of the 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 using the actual IP address of the destination and the
MAC address of the router. The MAC address for the router was learned
from the ARP table by using the IP address of that router. 
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