Chabot College
ELEC 99.05
Address Resolution Protocol
CISCO NETWORKING ACADEMY
ARP
• Layer 2 (MAC) address processing
– Performed in NIC hardware
– Fast
– Low CPU overhead
• Layer 3 (IP) address processing
– Performed in software
– Slow
– High CPU overhead
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ARP
• Problem:
Layer 3 knows the network address that a
message should be sent to...
But layer 3 has no ability to actually send a
message over the media!
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ARP
• To send, Layer 3 needs the services of
Layer 2.
• To use those services, there must be a way
to tie
Layer 3 (IP) addresses to
Layer 2 (MAC) addresses
CISCO NETWORKING ACADEMY
ARP
• Address Resolution Protocol, or ARP, is
used to “map” IP addresses to MAC
addresses.
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The ARP Table
CISCO NETWORKING ACADEMY
The ARP Table
• The ARP table is stored in area of RandomAccess Memory on each host.
• Such an area of memory is often called a
cache. The ARP table is often referred to as
an ARP cache.
• Entries in the ARP table “age out.” They are
removed from the table after a period of
inactivity.
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The ARP Table
• Examine your curriculum PC’s arp table
now.
• Start | Programs | MS-DOS Prompt
• At the prompt, type arp -a
CISCO NETWORKING ACADEMY
The ARP Table
• Examine your curriculum PC’s arp table
now.
• Start | Programs | MS-DOS Prompt
• At the prompt, type arp -a
Microsoft(R) Windows 98
(C)Copyright Microsoft Corp 1981-1998.
C:\WINDOWS>arp -a
CISCO NETWORKING ACADEMY
The ARP Table
• You may see one or more entries in your
PC’s arp table:
Microsoft(R) Windows 98
(C)Copyright Microsoft Corp 1981-1998.
C:\WINDOWS>arp -a
Interface: 192.168.0.2 on Interface 0x2000003
Internet Address
Physical Address
Type
192.168.0.1
00-a0-c5-e2-ad-b8 dynamic
C:\WINDOWS>
CISCO NETWORKING ACADEMY
The ARP Table
• Or you may find that your PC’s arp cache is
empty:
Microsoft(R) Windows 98
(C)Copyright Microsoft Corp 1981-1998.
• C:\WINDOWS>arp -a
No ARP Entries Found
• Why might it be empty?
• Aging...
CISCO NETWORKING ACADEMY
Aging Out
For Microsoft Windows hosts:
• Initial mappings have a 2-minute “time-to-live”.
• An entry that is used twice in 2 minutes is
automatically given a 10-minute time-to-live.
CISCO NETWORKING ACADEMY
ARP
• Problem:
The IP address of a host is known, but it’s
MAC address is not.
How can IP learn the MAC addresses of a
host?
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ARP
• Solution:
IP issues an ARP request
CISCO NETWORKING ACADEMY
Sending Messages
• The use of ARP depends on the message
destination:
– 1. Local destination - the source and
destination hosts are on the same
network segment.
– 2. Remote destination - source and
destination are on different network
segments.
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Sending Example: Local
From Network A, Host 1
To Network A, Host 3
Network A
Host1
Host2
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Network B
Host3
Host1
Sending Example: Local
From Network A, Host 1
To Network A, Host 3
Source & destination IP address are on the same
subnet:
Network A
Host1
Host2
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Network B
Host3
Host1
Sending Example: Local
Step One:
• Is the IP address in the ARP cache?
– If yes, use the cached MAC address to
address the message.
– If no, go to Step 2...
CISCO NETWORKING ACADEMY
Sending Example: Local
Step Two: (when there is no ARP entry)
• Send ARP request to find MAC address of
destination.
• The ARP Request is a layer-2 broadcast
frame. Therefore, each computer on the
network
– accepts the frame
– passes it up to layer 3 to determine if it is the
owner of the requested IP address.
CISCO NETWORKING ACADEMY
Sending Example: Local
Step Three: (performed by just one computer)
• ARP Reply is sent directly to the hardware
address of the requesting system.
Note - Only the owner of the requested IP
address will provide the ARP reply. All other
hosts disregard the request.
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Sending Example: Local
Step Four:
• Upon receiving the reply, the requesting
machine will add the address into its ARP
cache and use the newly discovered MAC
address to address the message.
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Sending Example: Remote
From Network A, Host 1
To Network B, Host 1
Network A
Host1
Host2
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Network B
Host3
Host1
Sending Example: Remote
From Network A, Host 1
To Network B, Host 1
Source & destination IP address are not on the same
subnet:
Network A
Host1
Host2
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Network B
Host3
Host1
Sending Example: Remote
Step One:
• Is the IP address local (same subnet)? No.
To leave a subnet or network, a computer
must use the services of a router.
Routers are sometimes called gateways for
this reason.
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Default Gateway
Default Gateway: the network interface (of a
router) to which the client sends traffic that is
destined for other networks.
Default
Gateway
Network A
Host1
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Host2
Host3
Other
Networks
Sending Example: Remote
Step Two:
• Sending computer checks for a default
gateway in its TCP/IP configuration.
• If no default gateway is installed, the
sending computer cannot send the
message.
CISCO NETWORKING ACADEMY
Sending Example: Remote
Step Three:
• Sending computer checks ARP table for IP
and MAC address of default gateway.
• If there is no ARP entry for the default
gateway, the sending computer sends an
ARP request, looking for the router.
CISCO NETWORKING ACADEMY
Sending Example: Remote
Step Four:
• Gateway router sends ARP Reply directly
to the hardware address of the requesting
system.
Note - Only the router will provide the ARP
reply. All other hosts disregard the request.
CISCO NETWORKING ACADEMY
Sending Example: Remote
Step Five:
• Sending computer addresses message
using non-local destination hosts’s IP, but
uses default gateway’s MAC address.
(The computer will always use the MAC
address of the default gateway when
addressing messages to hosts on a remote
IP network.)
CISCO NETWORKING ACADEMY
Sending Summary
• Local Destination...
– IP address - destination host
– MAC address - destination host
• Remote Destination...
– IP address - destination host
– MAC address - default gateway of
router
CISCO NETWORKING ACADEMY
Proxy ARP
• A rarely used variation of ARP.
• Used only between subnets of the same net.
• Hosts are configured to view all subnets as a single
network. This is typically done by configuring the
workstation with a smaller subnet mask than the
network really uses.
The hosts don’t know there is a router between them
and the destination host. The hosts don’t have a
default gateway defined.
• Proxy ARP occurs when the router is configured to
send an ARP reply to the requesting host on behalf of
the destination host. The router gives its own MAC
address in the ARP reply.
CISCO NETWORKING ACADEMY
Proxy ARP
• Exam question says:
In Proxy ARP, a router helps a device on one
subnet to find the MAC address of a host on
another subnet.
• This is the “correct” answer, but it isn’t true.
Actually the router gives its own MAC address.
• Rarely used.
Only important for the exam.
• For technical detail, follow this link...
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Router’s ARP table
• Usually bigger than workstation’s table
• Can have addresses from more than one
network
• Includes what interface or port that network
is connected to in the table
CISCO NETWORKING ACADEMY
Router’s ARP table
Protocol Address
Age
Internet 169.199.73.195
Internet 169.199.73.194
Internet 207.124.101.220
Internet 207.124.101.201
CISCO NETWORKING ACADEMY
(min) Hardware Addr
8
00e0.2908.398d
5
0000.0c77.01f4
15
00e0.24e7.bc43
2
00e0.ff42.0034
Interface
Ethernet0
Ethernet0
Ethernet1
Ethernet1
Duplicate IP Addresses
• How would ARP behave if two hosts were
accidentally assigned the same IP address?
Duplicate IP Addresses
Network A
Host1
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Host2
Host3
Host3
Host5
Duplicate IP Addresses
• Two MAC addresses would be mapped to the
same IP address - a forbidden condition!
Duplicate IP Addresses
Network A
Host1
Host2
Host3
Host3
ARP Reply ARP Reply
CISCO NETWORKING ACADEMY
Host5
ARP Quiz
• Sending from NetA Host1 to NetC Host 2
• Which MAC addresses must be known or
discovered with ARP?
Host1
Network C
Network B
Network A
Host2
From: Network A
Host1
Host 1
CISCO NETWORKING ACADEMY
Host2
Host1
To:Network C
Host2
Host 2
ARP Quiz
• Which MAC addresses must be known?
Default Gateway of Network A
Default Gateway
MAC
Host1
Network C
Network B
Network A
Host1
Host2
From: Network A
Host 1
CISCO NETWORKING ACADEMY
Host2
Host1
To:Network C
Host2
Host 2
ARP Quiz
• Which MAC addresses must be known?
Default Gateway of Network A
Default Gateway of Network B
Default Gateway
MAC
Network C
Network B
Network A
Host1
Default Gateway
MAC
Host1
Host2
From: Network A
Host 1
CISCO NETWORKING ACADEMY
Host2
Host1
To:Network C
Host2
Host 2
ARP Quiz
• Which MAC addresses must be known?
Default Gateway of Network A
Default Gateway of Network B
Host 2 on Network C
Default Gateway
MAC
Host1
Host2
From: Network A
Host 1
CISCO NETWORKING ACADEMY
Host
MAC
Network C
Network B
Network A
Host1
Default Gateway
MAC
Host2
Host1
To:Network C
Host2
Host 2