11-Understanding ARP Request

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Understanding ARP Request

UnderstandingARP Request Article covers the following CCNA/ICND1 Exam Topics:

Under Operation of IP Data Networks:

1. Identify common applications and their impact on the network.

2. Predict the data flow between two hosts across a network.

Recommended Study Plan:

1. Download the ICND1v2 Exam Topics Sheet from Cisco Website.

2. Follow the Steps and the Articles under IP Networks Fundamentals by order.

3. Or Start with Web Browser DNS LookupArticle.

Average Time Required Studying this Article: 1 Hour

ARP Protocol Process Mapping IP address to MAC Address

Address Resolution Protocol (ARP) used only on Ethernet Networks by Data Link Layerto provide two basic

functions:

Resolving IPv4 addresses to MAC addresses

Maintain a cache of MAC to IP address Mapping Table

Locally speaking inside the LAN, if a shared folder located in a server with IP 192.168.1.10 (using windows

environment) DNS will resolve human names such \shared-folder to server IP Address that host the shared folder, b

still the network need to resolve one more level to Data link address, and since ethernet standard is used , ARP is

called to resolve IP Addresses to MAC addresses only on Ethernet Networks.

Why ARP is needed?

IP routing logic requires that network devices encapsulate IP packets inside Data-Link frames based on the link typeused, and If Ethernet link type is used, then ARP resolution from IP Addresses to MAC addresses is required to

encapsulate IP packets inside Ethernet frames.

Whenever a host or a router needs to communicate with other hosts or routers, they encapsulate TCP or UDP

segments in an IP packet. Hosts and Routers Network Layer knows all the important pieces to build an IP packet

such destination IP address and source IP address, but in Ethernet Network and when it comes to encapsulating an

IP packet inside a frame using destination and source MAC addresses, hosts and routers do not know neighbors

NICs or interfaces MAC addresses beforehand, hence, ARP get evolved.

TCP/IP defines ARP as the method by which any host or router on a LAN can dynamically learn the MAC address o
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another host or router on the same LAN. ARP includes a protocol called ARP request, which is a message that asks

the simple request if this is your IP address, please reply with your MAC address . ARP also includes the ARP Rep

message, which indeed lists both the original IP address and the matching MAC address.

Detailed ARP Request and Reply Process

Based on the figure above, PC1s Ethernet cable plugged into SW2, the following steps take place to show how PC1

initially learned its Default Gateways (R3) MAC address in order to route an IP packets outside its LAN network.

Step 1:PC1s Network Layer sends an IP packet encapsulated with destination IP address of R3 (192.168.1.1) as a

receiver, and a source MAC address of PC1 (192.168.1.100) as sender.

Step 2:PC1s Data-Link Layer encapsulate the IP packet inside an Ethernet Frame with a broadcast destination MA

Address (FFFF.FFFF.FFFF), which means all devices on this LAN will receive this message , and a source MAC

address of PC1 NIC interface as sender.

Step 3:The switch receives the broadcast frame, learn the source MAC address and record it along the interface po

number in its MAC table (1), and flood the broadcast frame from all ports except the port where it was received (2).

Step 4:All devices receive the broadcast packet and drop it, except R3. R3 replies to PC1 with its MAC address sinc

the IP packet includedARP message requestindicating its IP address in the destination field.

Step 5:PC1 receives the ARP reply request indicating R3s MAC address, learn the MAC address and saves it in its

MAC table.

Step 6:PC1s Network layer is ready right now to create an IP packet heading to Web-server instead of R3, and

ready to encapsulate it inside an Ethernet Frame to be forward to R3 MAC address as next-hop.

Notice, how PC1 has to prepare the pieces before completing web page request, resolving Name to IP using DNS (1

learn R3 MAC address (2), and finally creates an HTTPGet request IP packet heading to a web-server.

Lets Dig Deeper into the Process

Note that R3 IP Address (192.168.1.1) known to PC1 as Default Gateway by DHCP or Static configuration. Thefollowing figure shows how PC1 created an IP packet indicating R3 IP address as destination, and encapsulated the

packet inside a frame indicating an ARP Requestas destination Broadcast address ( FFFF.FFFF.FFFF) inside the

frame.
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Once PC1 release a broadcast frame, it will hit all network devices on this LAN address which is 192.168.1.0/24. Aft

R3 receives the broadcast frame, replies with its MAC, and PC1 learn R3s MAC address, the next figure shows how

PC1 created a new IP packet indicating Web-Servers IP address this time, and encapsulated the IP packet inside a

Frame heading this time to R3 MAC address as next-hop instead of Broadcast Address since PC1 learned R3s MA

address already.

Note that Hosts save the ARP replies in their ARP cache or ARP table for future use as they do with DNS queries.

ARP is not used every time the host or the router needs to forward a packet to neighbor device. Each time a host or

router needs to send a packet encapsulated inside an Ethernet Frame, it first checks its ARP table for correct IP

address and matching MAC address. Hosts and routers will let ARP cache entries time out to clean up the table, so

occasionally ARP Requests traffic can be seen on LAN.

Unicast, Broadcast, and Multicast Frames


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Why we need different types of Ethernet frames? To understand how ARP service works, its good to look at the

difference among all 3 types of Ethernet frames communication. By looking at the figures below and for simplicity,

note how each type has different color. Unicast is Orangeand what inside the circle corresponds to the color as wel

and so on.

Unicast frame:indicates that only 2 network devices talking to each other.

(Remember Hubs dont care and they broadcast everything Unicast, Broadcast, and Multi-Cast. In contrast, switches

do care, why? Because they know how to talk Ethernet language, therefore, know how to deal with MAC addresses

any Ethernet frame type).

Broadcast frame:indicates that a single machine sending a frame to everyone on the LAN.


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Multicast frame:indicates that a group of devices are willing to respond to a specific machine request on the

network, or a single network device is trying to talk only to a specific group of devices that are willing to listen to a

specific Multicast frame that hold a MAC address such this one 0100.5E7F.0001.

And finally Little bit about ICMP why its needed for troubleshooting?


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Internet Control Message Protocol (ICMP)

Troubleshooting tools Use to verify connectivity such ping and traceroute commands.

ICMP Includes some primary tools for troubleshooting and verifyingbasic network connectivity, commands suc

Packet Internet Groper (Ping)and Traceroute(tracert in windows).

Pinguses ICMP to send a message called ICMP echo requestto another IP address. The host with that IP address

should reply with an ICMP echo reply packet. If that works, you successfully have tested the IP network. In another

words, you know that the network can deliver an IP packet from one host to the other and back. ICMP does not rely any application; it tests basic layer 1, 2, and 3 connectivity of TCP/IP model. The following shows a ping reply from a

host google.com.

Traceroute uses ICMP as well, is a network tool used to test and shows an IP packet route taken by PC1s Internet

Protocol(IP) across the routers that are located between PC1 and Web-server. E.g. the following shows a windows

tracert command from a host to google.com.

Detailed ICMP header
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Next: Understanding TCP and UDP Protocols

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About Imad Daou

He is the founder of CCNA HUB, a CCNA Training HUB to help CCNA students get certified. Imad has more than 10years of IT experience as Field Service and Consulting Engineer. A+, Network+, Server+, Security+, Storage+, HP,

Dell, and IBM Hardware Certified. He's a Professional SMB IT Consultant.

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11-Understanding ARP Request

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