Information Technology Reference
In-Depth Information
23.12 ARP
ARP (address resolution protocol) translates IP addresses to Ethernet addresses. This is used
when IP packets are sent from a computer, and the Ethernet address is added to the Ethernet
frame. A table look-up, called the ARP table, is used to translate the addresses. One column
has the IP address and the other has the Ethernet address. The following is an example ARP
table:
IP address Ethernet address
146.176.150.2 00-80-C8-22-6BE2
146.176.150.3 00-80-C8-22-CD4E
146.176.150.4 00-80-C8-23-114C
A typical conversation is as follows:
1. Application sends an application message to TCP.
2. TCP sends the corresponding TCP message to the IP module. The destination IP address
is known by the application, the TCP module, and the IP module.
3. At this point the IP packet has been constructed and is ready to be given to the Ethernet
driver, but first the destination Ethernet address must be determined.
4. The ARP table is used to look up the destination Ethernet address.
The sequence of determining the Ethernet address is as follows:
1.
An ARP request packet with a broadcast Ethernet address (
FF-FF-FF-FF-FF-FF
) is
sent out on the network to every computer. Other typical Ethernet broadcast addresses
are given in Section 23.16.3.
2.
The outgoing IP packet is queued.
3.
All the computers on the network segment read the broadcast Ethernet frame, and exam-
ine the Type field to determine if it is an ARP packet. If it is then it is passed to the ARP
module.
4.
If the IP address of a receiving station matches the IP address in the IP packet then it
sends a response directly to the source Ethernet address.
5.
The originator then receives the Ethernet frame and checks the Type field to determine if
it an ARP packet. If it is then it adds the sender's IP address and Ethernet address to its
ARP table.
6.
The IP packet can now be sent with the correct Ethernet address.
Each computer has a separate ARP table for each of its Ethernet interfaces.
23.13 IP multicasting
Many applications of modern communications require the transmission of IP datagrams to
multiple hosts. Typical applications are video conferencing, remote teaching and so on. This
is supported by IP multicasting, where a host group is identified by a single IP address. The
