Address Resolution - MODULO 9 Flashcards
MAC and IP
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.
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.
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.
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.
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.
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.
To send a packet to another host on the same local IPv4 network
a host must know the
IPv4 address and the MAC address of the destination device.
Device destination IPv4 addresses are either known or resolved by device name.
However, MAC addresses must be discovered.
A device uses Address Resolution Protocol (ARP) to determine the destination MAC
address of a local device when it knows its IPv4 address.
ARP provides two basic functions:
- Resolving IPv4 addresses to MAC addresses
- Maintaining a table of IPv4 to MAC address mappings
ARP Functions
When a packet is sent to the data link layer to be encapsulated into an Ethernet frame, the
device refers to a table in its memory to find the MAC address that is mapped to the IPv4
address.
This table is stored temporarily in RAM memory and called the ARP table or the ARP
cache.
The sending device will search its ARP table for a destination IPv4 address and a
corresponding MAC address.
- If the packet’s destination IPv4 address is on the same network as the source IPv4
address, the device will search the ARP table for the destination IPv4 address. - If the destination IPv4 address is on a different network than the source IPv4
address, the device will search the ARP table for the IPv4 address of the default
gateway.
In both cases
the search is for an IPv4 address and a corresponding MAC address for the
device.
Each entry, or row, of the ARP table binds an IPv4 address with a MAC address.
We call the relationship between the two values a map.
This simply means that you can locate an IPv4 address in the table and discover the
corresponding MAC address.
The ARP table temporarily saves (caches) the mapping for the devices on the LAN.
If the device locates the IPv4 address, its corresponding MAC address is used as the
destination MAC address in the frame.
If there is no entry is found, then the device sends an ARP request.
ARP Request
An ARP request is sent when a device needs to determine the MAC address that is
associated with an IPv4 address, and it does not have an entry for the IPv4 address in its
ARP table.
ARP messages are encapsulated directly within an Ethernet frame.
There is no IPv4 header.
The ARP request is encapsulated in an Ethernet frame using the following header
information:
- Destination MAC address – This is a broadcast address FF-FF-FF-FF-FF-FF
requiring all Ethernet NICs on the LAN to accept and process the ARP request. - Source MAC address – This is MAC address of the sender of the ARP request.
- Type - ARP messages have a type field of 0x806. This informs the receiving NIC
that the data portion of the frame needs to be passed to the ARP process.
Because ARP requests are broadcasts
they are flooded out all ports by the switch, except
the receiving port.
All Ethernet NICs on the LAN process broadcasts and must deliver the ARP request to its
operating system for processing.
Every device must process the ARP request to see if the target IPv4 address matches its
own.
A router will not forward broadcasts out other interfaces.
Only one device on the LAN will have an IPv4 address that matches the target IPv4
address in the ARP request. All other devices will not reply.
ARP Operation - ARP Reply
Only the device with the target IPv4 address associated with the ARP request will respond
with an ARP reply.
The ARP reply is encapsulated in an Ethernet frame using the following header information:
- Destination MAC address – This is the MAC address of the sender of the ARP
request. - Source MAC address – This is the MAC address of the sender of the ARP reply.
- Type - ARP messages have a type field of 0x806. This informs the receiving NIC
that the data portion of the frame needs to be passed to the ARP process.
Only the device that originally sent the ARP request will receive the unicast ARP reply.
After the ARP reply is received, the device will add the IPv4 address and the corresponding
MAC address to its ARP table.
Packets destined for that IPv4 address can now be encapsulated in frames using its
corresponding MAC address.
If no device responds to the ARP request, the packet is dropped because a frame cannot
be created.
Entries in the ARP table are time stamped.
If a device does not receive a frame from a particular device before the timestamp expires,
the entry for this device is removed from the ARP table.
Additionally, static map entries can be entered in an ARP table, but this is rarely done.
Static ARP table entries do not expire over time and must be manually removed.
Note: IPv6 uses a similar process to ARP for IPv4, known as ICMPv6 Neighbor Discovery
(ND). IPv6 uses neighbor solicitation and neighbor advertisement messages, similar to
IPv4 ARP requests and ARP replies.
ARP Role in Remote
Communications
When the destination IPv4 address is not on the same network as the source IPv4 address,
the source device needs to send the frame to its default gateway.
This is the interface of the local router.
Whenever a source device has a packet with an IPv4 address on another network, it will
encapsulate that packet in a frame using the destination MAC address of the router.
The IPv4 address of the default gateway is stored in the IPv4 configuration of the hosts.
When a host creates a packet for a destination, it compares the destination IPv4 address
and its own IPv4 address to determine if the two IPv4 addresses are located on the same
Layer 3 network.
If the destination host is not on its same network, the source checks its ARP table for an
entry with the IPv4 address of the default gateway.
If there is not an entry, it uses the ARP process to determine a MAC address of the default
gateway.
Removing Entries from an ARP Table
For each device, an ARP cache timer removes ARP entries that have not been used for a
specified period of time.
The times differ depending on the operating system of the device.
For example, newer Windows operating systems store ARP table entries between 15 and
45 seconds, as illustrated in the figure.
ARP Issues - ARP Broadcasts and
ARP Spoofing
As a broadcast frame, an ARP request is received and processed by every device on the
local network.
On a typical business network, these broadcasts would probably have minimal impact on
network performance.
However, if a large number of devices were to be powered up and all start accessing
network services at the same time, there could be some reduction in performance for a
short period of time, as shown in the figure.
After the devices send out the initial ARP broadcasts and have learned the necessary MAC
addresses, any impact on the network will be minimized.
In some cases, the use of ARP can lead to a potential security risk.
A threat actor can use ARP spoofing to perform an ARP poisoning attack.
This is a technique used by a threat actor to reply to an ARP request for an IPv4 address
that belongs to another device, such as the default gateway, as shown in the figure.
The threat actor sends an ARP reply with its own MAC address.
The receiver of the ARP reply will add the wrong MAC address to its ARP table and send
these packets to the threat actor.
