Home > Articles > Cisco > CCNA Routing and Switching

The Anatomy of a Packet Path (ICMP) from Source to Destination

Two of the most common tasks that are performed by a network engineer are using the ping and traceroute (tracert) utilities. This article takes a look at how the IP packet is structured and routed and then reviews how both the ping and traceroute (tracert) utilities use IP/ICMP packets to perform their functions.
Like this article? We recommend

Two of the most common tasks that are performed by a network engineer are using ping and using traceroute (tracert). An experienced engineer is able to use these commands to achieve an understanding of a number of different problems; these skills mainly come from an understanding of how the IP packet is structured and how traffic is routed. This article takes a look at how the IP packet is structured and routed, and then reviews how both the ping and traceroute (tracert) utilities use IP/ICMP packets to perform their functions.

IP Packet and IP Packet Routing

Before the analysis of a packet path can really begin, some amount of knowledge in how an IP packet is structured is required. Figure 1 below shows the fields that exist within the IPv4 header.

Figure 1 IPv4 Header

When using the ping utility, the engineer needs to have a basic idea of how a packet is routed from a source to destination; as the details of IP routing are contained within many pages of various books, this article will only cover the basic idea of IP routing. When an IP packet is sent from a source to destination there are a few steps that are taken to determine how exactly the sending devices treats the packet. If the packet is destined for another IP address within the same subnet, the packet will simply be sent using Layer 2 techniques (i.e. ARP). If the packet is destined to a separate subnet, the device will review its existing routing table for a specific known path to the intended destination. If this route exists, the packet is sent along this known path; if a specific route is not known, a default route is used, if it exists; if neither of these exist then the packet is dropped and an error given. (What and how the ping utility operates is covered in more detail in the What is a Ping section.) The traceroute utility also requires an understanding of IP routing but also knowledge of how the Time to Live (TTL) field is used within the IP packet; further information about the mechanics of how traceroute works will be discussed in the What is Traceroute section.

What is a Ping?

At its most basic, the ping command is a utility that is used to send 4 or 5 Internet Control Message Protocol (ICMP) packets from a source to a destination using a small buffer size and a simple payload (on Microsoft it is a 32-byte payload with contents being the alphabet from ‘a’ through ‘w’ repeating). Ping itself is a very simple command that can help an engineer figure out a number of things. For example, if the ping is successful, then the path from source to destination is working with no problems; if there is a failure, it tells the engineer that it is not working. An engineer also has the ability to change the ping parameters. For example, they could troubleshoot packet size issues along the path by changing the buffer size or verify working interfaces by changing the number of packets sent to the destination to determine interface consistency. These are just a few of the situations where the ping utility can be a valuable asset to a network engineer.

What is Traceroute?

As stated above, traceroute (Tracert) utilizes the TTL field within the IPv4 header. The TTL field provides the ability to limit the number of hops that will be taken by a specific IP packet. For example, if a packet is never to go over one hop away from a source then the TTL will always be set to 1. The traceroute utility uses this mechanism to find the specific path that a packet is taking from source to destination. The first packet will be sent out using a TTL of 1; if it reaches the destination then a response will tell the utility to stop continuing to look for further hops. If the destination is not reached, the next packet will be sent with a TTL of 2, and this process will repeat until the destination is reached; the responses that are returned from each iteration are compiled by the utility to visualize the path from source to destination. Knowing what path a packet is taking from a source to destination can be used for a number of different troubleshooting situations.

Ping Packet Analysis

In this section, we’ll take a look at the specific traffic that is sent from the ping utility to solidify the ideas covered above, by showing a few figures that display captures taken of ICMP packets sent by the ping utility. Figure 2 shows the capture of the initial ICMP request packet that is sent to a destination; this capture is using the ping utility provided by Microsoft Windows which sends four ICMP requests.

Figure 2 ICMP Request

As can be seen from the capture, the ping command issued was ping with the default settings (4 requests with a 32 byte payload). Figure 3 shows the first ICMP reply from the destination that verifies that the destination is reachable.

Figure 3 ICMP Reply

As is shown, this Request/Reply is completed four times which confirms a consistent path from the source to destination.

Traceroute (tracert) Packet Analysis

In line with what was covered in the Ping Packet Analysis section, this section shows a few figures of ICMP packets sent using the tracert Windows utility. It is, however, important to point out that the operation of the tracert Windows utility and the traceroute Linux and Cisco IOS utilities is slightly different; the tracert utility utilizes ICMP to perform a trace of a route from source to destination, while the traceroute utility utilities UDP/IP. This can be confusing in some circumstances because many network devices automatically will drop ICMP requests due to security concerns; this makes the results from the tracert utility look inconclusive. However, most of the same network devices will allow the UDP traffic which can make the results from the traceroute utility look more complete.

Figure 4 below shows the initial ICMP packet that is sent from the command tracert

Figure 4 Traceroute (tracert) Hop 1

The captures show that this packet is sent using a TTL value of 1 and a TTL exceeded message is returned telling the source device that the destination has not been reached yet. Another thing that should be noticed is that by default, the tracert utility sends three identical ICMP request packets (using TTL of 1); this is to gauge the response time from the source to each hop. Figure 5 shows the first packet that is sent using a TTL of 2 with a following ICMP reply, which tells the source device that the destination was reached.

Figure 5 Traceroute(tracert) Hop 2


With just a little knowledge of how IP/ICMP operates a wealth of information can be obtained and used in a number of situations. These simple utilities are used by every level of network engineer from the new engineers just getting their feet wet to the most experienced network architects; any upcoming network engineers should ensure that they are familiar with them, how they work and how they can be used. Hopefully you now have enough information to get started in this endeavor and will help in future situations.

Pearson IT Certification Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Pearson IT Certification and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Pearson IT Certification products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by Adobe Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.pearsonitcertification.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020