Understanding the network.pdf - Back to Home

• First, it performs translations when a host needs to send data to an IP
address for which it has no Ethernet address.
• Second, it keeps track of the hosts it discovers during its broadcasts.
Here's how it works, step by step:
1. The host makes an ARP request. The ARP request is a broadcast to all the
locally connected stations, asking, "What is the Ethernet address for
X.X.X.X?"
2. Destination hosts reply to the ARP request. The request is made using the
first IP datagram destined for delivery. Because the datagram is busy asking,
the data is lost during the ARP request process, so the datagram must be
retransmitted. This retransmission is usually unseen by the user, but it can
be demonstrated when a new host is connected to the network and a user
pings another local host.
3. If the destination host is not in the user host's ARP cache, an ARP request is
made, usually resulting in the loss of the first datagram. But, ARP also keeps
a record of the Ethernet/IP address translations it discovers. These
mappings are stored in RAM until the host is shut down.
In situations where the host knows little other than its Ethernet address at startup,
RARP is used. RARP is an IP level client/server address provisioning service.
When the "dumb" host boots up, it sends a RARP broadcast across the local network
segment. A RARP server that knows the IP address that corresponds to the Ethernet
address of the requesting station responds to the request and provides the IP
address to the host. This process only occurs at boot time.
NOTE
The lifetime of an ARP entry depends on a hardware platform's IP implementation.
For example, Cisco routers flush the ARP table entries approximately every four
hours.
Sometimes, these default purges are not fast enough. In a case where you need to
change a bad Network Interface Card (NIC) on a host, when you first bring the host
back online, you might not be able to exchange data with other hosts on the
network. When this happens, the first thing you should do is flush the ARP cache on
the hosts to which you are having trouble connecting. Corrupt and incorrect ARP
entries are the usual suspects in situations where intermittent or sudden lapses in
connectivity happen between locally connected hosts.