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TechKnowSurge Catalog Videos What’s a Packet?
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What’s a Packet?

An IP packet combines a data payload with a header containing source and destination IP addresses, enabling messages to be routed across networks. Understanding packet structure is foundational to how IP-based communication works.

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About this video

An IP packet is the core building block of networked communication, formed by attaching a header to a data payload. That header contains a source IP address identifying the sender and a destination IP address identifying the recipient, along with 96 bits of additional fields that support routing and delivery. The payload itself, the actual message or data being transmitted, remains unchanged as the packet moves across networks toward its destination. Because the header provides clear addressing information, IP packets are routable across vast distances, from one side of a network to the other side of the planet. The structure stays relatively consistent throughout that journey, which is why the term packet is used specifically in the context of IP communication, though it is also applied more loosely in general networking discussions. A key distinction exists between an IP packet and an Ethernet frame. When a packet needs to be delivered over a local network segment, it is encapsulated inside a frame, which provides the addressing and structure required for that shorter, local hop. The packet rides inside the frame for local delivery, but the packet itself remains intact, ready to continue its routed path once it moves beyond the local network.

What you'll learn

What's covered

IP Packets

Aligned to

CompTIA Network+
1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.
Cisco CCNA
1.6 Configure and verify IPv4 addressing and subnetting
1.13 Describe switching concepts
Cisco CyberOps Associate
4.8 Interpret the fields in protocol headers as related to intrusion analysis
4.3 Compare deep packet inspection with packet filtering and stateful firewall operation
Cisco CCST Networking
1.1 Identify the fundamental conceptual building blocks of networks
Cisco CCST Cybersecurity
2.1 Describe TCP/IP protocol vulnerabilities

Key terms

Internet Protocol
IP
The principal communications protocol for routing packets across network boundaries.
IP Address
A numerical label assigned to each device connected to a network that uses the Internet Protocol.
Packet
A unit of data formatted for transmission over a network, containing a header, payload, and sometimes a trailer.
Payload
The actual data content of a packet or message, excluding headers and metadata.
Encapsulation
The process of wrapping data with protocol headers as it passes down the layers of the OSI model.
IP Packet Header
The portion of an IP packet that contains control information, including the source and destination IP addresses, prepended to the payload.
Ethernet Frame
The Layer 2 data unit used on Ethernet networks, containing a preamble, source and destination MAC addresses, an EtherType field identifying the Layer 3 protocol, a payload (typically an IP packet), and a 4-byte FCS for error detection. Switches use the MAC addresses in Ethernet frames to make forwarding decisions.

Transcript

An IP address allows us to send a message clear across this planet, but we need to attach that IP address to the message, and we do that with a header, and we call that a packet.

What an IP Packet Looks Like

Here's what an IP packet looks like. Now, it's not my intention to get real in-depth into this, but here is the payload part of this. This is the message that's being delivered. And all we add, we don't have a trailer, we just add a header to this. And the header has a source IP address, where it's coming from, and a destination IP address, where it's heading to, where this is destined to. There is also just some information that's added into it. There's actually a lot of information, as we see. There's 96 bits of information added at the beginning, and they all have different purposes. So once again, I'm not going to get in depth into that, but the key part of this is that now we have a source and a destination IP address to help it deliver this message to wherever it needs to go.

So now we have that IP address that we want to send this message to, and we have our message, our data here, and we want to send it to 48.51.46.55. So we put that in the destination IP address, we put our own IP address in the source, and then we send it along its way. And now it's routable to that location of wherever it needs to go. So this whole thing is called the packet, and the packet remains relatively the same as it goes along its way. And so a lot of times you'll hear the term packet as very specific to IP, and a lot of times we'll use it just in more of a general sense as well.

Packets Versus Frames

Now, I don't want to get too much into all of the different layers, but I thought I would just mention the difference between an IP packet and an Ethernet frame, so we're not getting off on the wrong track as far as thinking of this correctly. An IP packet looks like this. It has the information, the source IP, the destination IP, and the payload to it, and it largely remains the same as it gets routed clear across this planet. However, for local delivery of this packet, we then encapsulate it into a frame. So this is a frame and this is a packet, and we put the packet into the frame so it can be delivered on the local network.

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