Address Resolution
Protocol
(ARP)
TCP/IP Protocol Suite
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reproduction or display.
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ADDRESS MAPPING
The delivery of a packet to a host or a router
requires two levels of addressing: logical and
physical. We need to be able to map a logical
address to its corresponding physical address and
vice versa. These can be done using either static
(Static mapping means creating a table that associates a logical address with
or Dynamic
a physical address)
mappingm (each time a
machine knows the logical address of another machine, it can use a protocol
to find the physical address. Two protocols have been designed to per-form
dynamic mapping: Address Resolution Protocol (ARP) and Reverse Address
Resolution Protocol (RARP)
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ADDRESS MAPPING
Anytime a host or a router has an IP datagram to
send to another host or router, it has the logical (IP)
address of the receiver. But the IP datagram must
be encapsulated in a frame to be able to pass
through the physical network. This means that the
sender needs the physical address of the receiver.
A mapping corresponds a logical address to a
physical address. ARP accepts a logical address
from the IP protocol, maps the address to the
corresponding physical address and pass it to the
data link layer.
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Position of ARP in TCP/IP protocol suite
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ARP operation
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ARP packet
16bit
8bit
16bit
8bit
16bit
Protocol type: IPv4 protocol is 0800 16
Target hardware address For an ARP request message, this field is all 0s because the sender
does not know the physical address of the target
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Encapsulation of ARP packet
An ARP packet is encapsulated directly
into a data link frame. For example, in
Figure below
an ARP packet is encapsulated in an
Ethernet frame. Note that the type field
indicates that the data carried by the
frame is an ARP packet.
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Encapsulation of ARP packet
Type: 0x0806
Preamble
and SFD
Destination
address
Source
address
Type
8 bytes
6 bytes
6 bytes
2 bytes
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Data
CRC
4 bytes
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Note
An ARP request is broadcast;
an ARP reply is unicast.
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Figure 8.5
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Four cases using ARP
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Example
A host with IP address 130.23.43.20 and physical address
B2:34:55:10:22:10 has a packet to send to another host with IP
address 130.23.43.25 and physical address A4:6E:F4:59:83:AB.
The two hosts are on the same Ethernet network. Show the ARP
request and reply packets encapsulated in Ethernet frames.
Solution
Figure shows the ARP request and reply packets. Note that the
ARP data field in this case is 28 bytes, and that the individual
addresses do not fit in the 4-byte boundary. That is why we do
not show the regular 4-byte boundaries for these addresses.
Also note that the IP addresses are shown in hexadecimal.
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Proxy ARP
Request
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Original cache table
RESOLVED
PENDING
FREE
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Dynamic Host Configuration
Protocol
DHCP
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©The McGraw-Hill Companies, Inc., 2000
INTRODUCTION
Each computer that uses the TCP/IP protocol suite
needs to know its IP address. If the computer
uses classless addressing or is a member of a
subnet, it also needs to know its subnet mask.
Most computers today need two other pieces of
information: the address of a default router to be
able to communicate with other networks and
the address of a name server to be able to use
names instead of addresses as we will see .
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DHCP OPERATION
The DHCP client and server can either be on the
same network or on different networks. Let us
discuss each situation separately.
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Client and server on the same network
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Client and server on two different
networks
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Use of UDP ports
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DHCP packet format
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Flag format
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Option format
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CONFIGURATION
The DHCP has been devised to
provide static and dynamic address
allocation.
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DHCP overview
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host broadcasts “DHCP discover” msg
DHCP server responds with “DHCP offer”
msg
host requests IP address: “DHCP request”
msg
DHCP server sends address: “DHCP ack”
msg
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Option with tag 53
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DHCP client transition diagram
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Exchanging messages
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DHCP Server

Start
Control Panel
Add/Remove Programs
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