Graphics Programs Reference
In-Depth Information
the two addressing schemes. In the office, post office mail sent to an
employee at the office's address goes to the appropriate desk. In Ethernet,
the method is known as Address Resolution Protocol (ARP).
This protocol allows “seating charts” to be made to associate an IP address
with a piece of hardware. There are four different types of ARP messages, but
the two most important types are
ARP request messages
and
ARP reply messages
.
Any packet's Ethernet header includes a type value that describes the packet.
This type is used to specify whether the packet is an ARP-type message or an
IP packet.
An ARP request is a message, sent to the broadcast address, that contains
the sender's IP address and MAC address and basically says, “Hey, who has
this IP? If it's you, please respond and tell me your MAC address.” An ARP
reply is the corresponding response that is sent to the requester's MAC address
(and IP address) saying, “This is my MAC address, and I have this IP address.”
Most implementations will temporarily cache the MAC/IP address pairs
received in ARP replies, so that ARP requests and replies aren't needed for
every single packet. These caches are like the interoffice seating chart.
For example, if one system has the IP address 10.10.10.20 and MAC
address
00:00:00:aa:aa:aa
, and another system on the same network has
the IP address 10.10.10.50 and MAC address
00:00:00:bb:bb:bb
, neither
system can communicate with the other until they know each other's MAC
addresses.
ARP request
Source MAC:
Dest MAC:
00:00:00:aa:aa:aa
ff:ff:ff:ff:ff:ff
“Who has 10.10.10.50?”
First system
Second system
IP:
MAC:
IP:
MAC:
10.10.10.20
10.10.10.50
00:00:00:aa:aa:aa
00:00:00:bb:bb:bb
ARP reply
Source MAC:
Dest MAC:
00:00:00:bb:bb:bb
00:00:00:aa:aa:aa
“10.10.10.50 is at
00:00:00:bb:bb:bb
.”
If the first system wants to establish a TCP connection over IP to the
second device's IP address of 10.10.10.50, the first system will first check its
ARP cache to see if an entry exists for 10.10.10.50. Since this is the first time
these two systems are trying to communicate, there will be no such entry, and
an ARP request will be sent out to the broadcast address, saying, “If you are
10.10.10.50, please respond to me at
00:00:00:aa:aa:aa
.” Since this request
uses the broadcast address, every system on the network sees the request, but
only the system with the corresponding IP address is meant to respond. In this
case, the second system responds with an ARP reply that is sent directly back
to
00:00:00:aa:aa:aa
saying, “I am 10.10.10.50 and I'm at
00:00:00:bb:bb:bb
.”
The first system receives this reply, caches the IP and MAC address pair in its
ARP cache, and uses the hardware address to communicate.
