CCNA 1 v7.0 Curriculum: Module 9 - Address Resolution
Apr 1,2020 Last Updated: May 2, 2023 CCNA v7.0, CCNA v7 Course #1 No Comments
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Contents
9.0. Introduction
9.0.1. Why should
I
take this module?
9.0.2. What will
I
learn to do in this module?
9.1. MAC and IP
9.1.1. Destination on Same Network
9.1.2. Destination on Remote Network
9.1.3. Packet Tracer - Identity MAC and IP Addresses
9.2. ARP
9.2.1. ARP Overview
9.2.2. ARP Functions
9.2.3. Video - ARP Request
9.2.4. Video - ARP Operation
-
ARP Reply
9.2.5. Video - ARP Role in Remote Communications
9.2.6. Removing Entries from an ARP Table
9.2.7. ARP Tables on Networking Devices
9.2.8. ARP Issues - ARP Broadcasts and ARP Spoofing
9.2.9. Packet Tracer - Examine the ARP Table
9.3. IPv6 Neighbor Discovery
9.3.1. Video - IPv6 Neighbor Discovery
9.3.2. IPv6 Neighbor Discovery Messages
9.3.3. IPv6 Neighbor Discovery - Address Resolution
9.3.4. Packet Tracer - IPv6 Neighbor Discovery
9.4. Module Practice and Quiz
9.4.1. What did I learn in this module?
9.4.2 Module Quiz - Address Resolution

9.0. Introduction
9.0.1. Why should I take this module?
Welcome to Address Resolution!
Hosts and routers both create routing tables to ensure that they can send and receive data
across networks. So how does this information get created in a routing table? As a network
administrator, you could enter these MAC and IP addresses manually. Butthat would take a
lot of time and the likelihood of making a few mistakes is great. Are you thinking that there
must be some way that this could be done automatically, by the hosts and routers
themselves? Of course, you are correct! And even though it is automatic, you must still
understand how this works, because you may have to troubleshoot a problem, or worse, your
network could be attacked by a threat actor. Are you ready to learn about address resolution?
This module has several very good videos to help explain the concepts, as well as three
Packet Tracer activities to cement your understanding. Why wait?
9.0.2. What will I learn to do in this module?
Module Title: Address Resolution
Module Objective: Explain how ARP and ND enable communication on a network.

Topic Title Topic Objective
MAC and IP Compare the roles of the MAC address and the IP address.
ARP Describe the purpose of ARP.
Neighbor Discovery Describe the operation of IPv6 neighbor discovery.
9.1. MAC and IP
9.1.1. Destination on Same Network
Sometimes a host must send a message, but it only knows the IP address of the destination
device. The host needs to know the MAC address of that device, but how can it be
discovered? That is where address resolution becomes critical.
There are two primary addresses assigned to a device on an Ethernet LAN:
• Physical address (the MAC address) - Used for NIC to NIC communications on
the same Ethernet network.
• Logical address (the IP address) - Used to send the packet from the source
device to the destination device. The destination IP address may be on the same
IP network as the source or it may be on a remote network.
Layer 2 physical addresses (i.e., Ethernet MAC addresses) are used to deliver the data link
frame with the encapsulated IP packet from one NIC to another NIC that is on the same
network. If the destination IP address is on the same network, the destination MAC address
will be that of the destination device.
Consider the following example using simplified MAC address representations.

192. 168.10.10/24 192.168.1 0.0/24 192.168.10.11/24
Destination MAC Source MAC Source IPv4 Destination IPv4
55-55-55 aa-aa-aa 192.168.10.10 192.168. 10.11
In this example, PC1 wants to send a packet to PC2. The figure displays the Layer 2
destination and source MAC addresses and the Layer 3 IPv4 addressing that would be
included in the packet sent from PC1.
The Layer 2 Ethernet frame contains the following:
• Destination MAC address - This is the simplified MAC address of PC2, 55-55-55.
• Source MAC address - This is the simplified MAC address of the Ethernet NIC on
PC1, aa-aa-aa.
The Layer 3 IP packet contains the following:
• Source IPv4 address - This is the IPv4 address of PC1, 192.168.10.10.
• Destination IPv4 address - This is the IPv4 address of PC2, 192.168.10.11.
9.1.2. Destination on Remote Network
When the destination IP address (IPv4 or IPv6) is on a remote network, the destination MAC
address will be the address of the host default gateway (i.e., the router interface).
Consider the following example using a simplified MAC address representation.

MAC
Scotdi
MAC
OXInqtiqtt
IPv4
DC-b& t C |■, aa-ea aa | 1 9 2 . ' U 3 / U . 1 O 1 1 0 . 1 . 1 .IU
In this example, PC1 wants to send a packet to PC2. PC2 is located on remote network.
Because the destination IPv4 address is not on the same local network as PC1, the
destination MAC address is that of the local default gateway on the router.
Routers examine the destination IPv4 address to determine the best path to forward the IPv4
packet. When the router receives the Ethernet frame, it de-encapsulates the Layer 2
information. Using the destination IPv4 address, it determines the next-hop device, and then
encapsulates the IPv4 packet in a new data link frame for the outgoing interface.
In our example, R1 would now encapsulate the packet with new Layer 2 address information
as shown in the figure.
DcatinaUon
MAC MAC IPv4 Deslmatton
IPV4
d c - O d U - J || CC-CC-Ct 1 ' 9 2 . 10.10 U’. 1.1.10 |
The new destination MAC address would be that of the R2 G0/0/1 interface and the new
source MAC address would be that of the R1 G0/0/1 interface.
Along each link in a path, an IP packet is encapsulated in a frame. The frame is specific to the
data link technology that is associated with that link, such as Ethernet. If the next-hop device

is the final destination, the destination MAC address will be that of the device Ethernet NIC.
as shown in the figure.
How are the IP addresses of the IP packets in a data flow associated with the MAC
addresses on each link along the path to the destination? For IPv4 packets, this is done
through a process called Address Resolution Protocol (ARP). For IPv6 packets, the process
is ICMPv6 Neighbor Discovery (ND).
9.1.3. Packet Tracer - Identify MAC and IP Addresses
In this Packet Tracer, activity you will complete the following objectives:
• Gather PDU Information for Local Network Communication
• Gather PDU Information for Remote Network Communication
This activity is optimized for viewing PDUs. The devices are already configured. You will
gather PDU information in simulation mode and answer a series of questions about the data
you collect.
9.1.3 Packet Tracer - Identify MAC and IP Addresses

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9.2. ARP
9.2.1. ARP Overview
If your network is using the IPv4 communications protocol, the Address Resolution Protocol,
or ARP, is what you need to map IPv4 addresses to MAC addresses. This topic explains how
ARP works.
Every IP device on an Ethernet network has a unique Ethernet MAC address. When a device
sends an Ethernet Layer 2 frame, it contains these two addresses:
• Destination MAC address - The Ethernet MAC address of the destination device
on the same local network segment. If the destination host is on another network,
then the destination address in the frame would be that of the default gateway (i.e.,
router).
• Source MAC address - The MAC address of the Ethernet NIC on the source
host.
The figure illustrates the problem when sending a frame to another host on the same segment
on an IPv4 network.

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