Certified - CompTIA Network +

In most network troubleshooting scenarios, problems can be identified and resolved using simple techniques: check the link lights, test connectivity, verify IP configuration, or confirm switch port settings. But what happens when all those checks come back clean, and yet the problem persists? That’s where deep packet inspection and traffic analysis come in. These tools allow you to capture, review, and dissect actual traffic as it moves through the network—examining not just whether data flows, but exactly how it behaves at every layer. When basic diagnostics fall short, packet capture becomes your most powerful tool for identifying the root cause of complex or persistent issues.
In this episode, we’ll explore how packet analysis tools like Wireshark, tcpdump, and NetFlow help you analyze network behavior at a granular level. You’ll learn how to capture traffic, filter for relevant packets, identify handshake problems, and detect latency, packet loss, and even application-layer issues. Whether you’re troubleshooting slow file transfers, VoIP quality drops, or strange application behavior, packet inspection gives you direct visibility into the network conversation. These skills are not only vital for advanced real-world troubleshooting—they’re frequently tested on the Network Plus exam as well.
So what exactly is packet capture? At its core, it’s the process of collecting raw traffic as it moves across the network. Each packet includes data such as source and destination IP addresses, port numbers, protocol headers, payload data, and flags. Capturing this information allows technicians to replay, decode, and analyze conversations between devices. This is especially useful when trying to figure out why a certain connection fails, why latency is increasing, or why some services behave inconsistently. Packet capture requires access to an interface—either directly on a device or via a mirrored or tapped connection.
Several tools are available for packet capture. The most popular is Wireshark, a free and open-source graphical tool that provides an intuitive interface for analyzing captured traffic. tcpdump is a command-line utility that runs on Unix-like systems and is ideal for lightweight or remote captures. TShark is the terminal-based counterpart to Wireshark and supports scripting for automated tasks. All of these tools save captures in PCAP format, which can be shared or revisited for further analysis. Each tool has strengths depending on the environment, but all offer deep insight into protocol behavior and data flow.
Using Wireshark for troubleshooting starts with capturing the right traffic. You can filter by IP address, protocol, port, or even specific application signatures. Once the traffic is captured, Wireshark displays each packet with the ability to drill down into the frame, packet, and segment layers. This level of visibility allows you to track TCP conversations, examine headers for anomalies, and check flags for handshake behavior. You can see if a session is being reset prematurely, if retransmissions are occurring, or if application responses are delayed. All of this information helps isolate the exact point of failure in the communication process.
To capture traffic without disrupting production systems, most technicians use SPAN or port mirroring. This feature, available on most managed switches, duplicates all traffic from one port or VLAN and sends it to another port for monitoring. You can connect a laptop running Wireshark to the mirrored port and observe the full flow of traffic, including handshake attempts, dropped connections, or application-specific anomalies. SPAN allows non-intrusive analysis, meaning you can inspect live network behavior without inserting additional hardware or interrupting the data stream.
For higher-fidelity inline capture, tap devices are often used. These are physical hardware splitters that sit between two network devices and send a passive copy of all traffic to a monitoring port. Taps are commonly used in security appliances or in data center environments where complete packet visibility is needed without interfering with production. Unlike SPAN ports, which can drop packets under heavy load, taps provide a full-duplex copy of traffic, ensuring that no frames are lost during analysis. These are ideal for use in intrusion detection systems or when auditing high-performance links.
In some cases, full packet capture isn’t necessary or practical. That’s where flow monitoring tools like NetFlow and sFlow come in. These protocols, supported by many routers and switches, summarize traffic behavior by reporting on flow-level statistics—such as which IP talked to which other IP, how many bytes were transferred, and which protocols or ports were used. While they don’t show payload data, flow tools are excellent for identifying traffic patterns, usage trends, or potential anomalies. They’re useful for performance monitoring, security auditing, and capacity planning.
One of the most useful things you can analyze in a packet capture is the TCP three-way handshake. This is the process by which a TCP connection is established: the client sends a SYN, the server responds with a SYN-ACK, and the client replies with an ACK. If this sequence fails, the connection never completes. If a reset (RST) occurs instead of an ACK, it often means the server rejected the connection or timed out. Analyzing this handshake allows you to verify whether sessions are completing and helps identify firewall blocks, port misconfigurations, or dropped packets.
Packet capture is also essential for diagnosing latency and packet loss. For example, if you observe repeated duplicate ACKs, it likely means the sender is not receiving the expected acknowledgment and is retransmitting. This could point to congestion, a faulty network segment, or poor signal quality. Wireshark allows you to track round-trip time (RTT), see sequence number gaps, and spot retransmission events—key indicators of degraded network performance. By comparing these metrics across multiple sessions, you can pinpoint whether a problem is affecting all traffic or isolated to certain paths or devices.
Another strength of packet capture is its ability to reveal application-layer issues. You can identify DNS failures by spotting requests with no replies or with error codes like NXDOMAIN. You can observe HTTP errors such as 404 or 503 returned from web servers, or track VoIP jitter, codec negotiation failures, and dropped RTP streams in voice traffic. These insights are especially useful when users report that an application “just doesn’t work,” but everything looks fine on the surface. By analyzing the application layer directly, you can distinguish between server-side, client-side, and network-level problems.
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Once a packet capture is complete, the next step is to extract the most relevant data. This is where filtering and search techniques become essential. Capturing thousands or even millions of packets without the ability to isolate specific flows quickly becomes unmanageable. Tools like Wireshark include powerful display filters that allow you to zero in on a single conversation, application, protocol, or error condition. You can filter by IP address, TCP port, MAC address, or even layer 7 fields such as DNS query names or HTTP status codes. For example, typing ip.addr == 192.168.1.10 shows all traffic to and from that device. Combining filters, such as tcp.port == 443 && !ip.addr == 192.168.1.1, lets you narrow the view even further. Effective filtering makes your analysis faster, more accurate, and far less overwhelming.
Sometimes, the capture needs to be reviewed by someone else—perhaps a vendor, another technician, or a senior engineer. In these cases, exporting and sharing the capture in PCAP format allows others to analyze the traffic using the same tools. Before sharing, however, it's important to review the data for sensitive information. Captures may contain usernames, passwords, private IP addresses, or internal domain names. If needed, Wireshark supports exporting filtered versions of captures or using the editcap utility to remove specific frames. Whenever sharing capture files, ensure that secure transfer and data retention policies are followed to protect the contents from unauthorized access.
Legal and ethical considerations play a major role in packet capture, especially in production environments. Capturing data from a live network may expose sensitive content such as login credentials, email messages, or proprietary application traffic. This makes it critical to obtain proper authorization and comply with organizational policy before starting any capture. In regulated industries, like healthcare or finance, unauthorized capture may constitute a data breach or compliance violation. Always ensure that any collection is justified, documented, and properly secured. Once captures are complete, store them in protected folders and limit access to authorized personnel only.
You’ll also need to decide whether to perform real-time or stored capture. Real-time analysis is useful for immediate troubleshooting—such as watching a connection fail as the user reproduces the problem. You can see the packets as they occur, identify issues in progress, and interact with the network directly. Stored captures, on the other hand, are helpful when problems are intermittent or occur overnight. These long-duration captures can be analyzed after the fact, using timestamps to correlate with reported issues. However, larger captures require more filtering and may need to be broken up or summarized before they become usable.
Packet analysis can also help verify and troubleshoot firewall rules. Firewalls drop or allow traffic based on rule sets, but sometimes it’s not clear whether a packet was allowed through or blocked silently. By capturing traffic on both sides of a firewall—or analyzing the logs alongside the capture—you can identify whether the SYN packet reached the destination, whether an ACK was returned, or whether a reset was issued. This helps determine if the firewall rule is correct or too restrictive. Packet traces also show attempts to use blocked ports or forbidden protocols, allowing security teams to adjust rules accordingly.
Intrusion detection systems (IDS) and intrusion prevention systems (IPS) rely heavily on packet inspection as part of their detection logic. These systems analyze traffic for known threat signatures, behavioral anomalies, or protocol violations. When a suspicious pattern is detected—such as a known malware command and control beacon or a port scan—the IDS logs an alert. An IPS can go further and drop the traffic in real time. While these systems are automated, they complement manual tools like Wireshark by highlighting suspicious flows that deserve deeper inspection. IDS and IPS alerts can also be used to filter large captures for post-event analysis.
For the Network Plus exam, expect to encounter questions where packet capture data is presented in simplified form. You may see a snippet showing a SYN packet with no ACK or a DNS request with an empty response. You'll be asked to interpret the situation, such as diagnosing a routing problem, firewall block, or DNS failure. Other questions may ask you to select the correct tool for a given scenario—such as using tcpdump on a headless server or deploying a span port on a managed switch. Familiarity with tool capabilities and how protocol flows behave in real life will help you choose the right response quickly and accurately.
In summary, packet and flow analysis offer unmatched visibility into the real behavior of your network. Capturing traffic allows you to verify whether systems are communicating correctly, whether protocols are functioning as expected, and whether applications are responding properly. Filtering and analysis tools reduce the noise and bring focus to the exact issue. While packet analysis can seem intimidating at first, with practice it becomes an indispensable skill in the network technician’s toolbox. Whether confirming the TCP handshake, investigating an application-layer timeout, or validating a firewall rule, packet capture brings clarity to complex problems.
When all else fails—when tools, logs, and dashboards haven’t explained the problem—packet inspection reveals the truth. It shows not what we think is happening, but what is actually occurring at the wire level. It’s a skill that blends precision with intuition, requiring both technical knowledge and investigative thinking. With the right tools, the right filters, and the right mindset, you can dissect traffic to its core and uncover the hidden causes behind connectivity issues, slowdowns, and service failures.