A packet header is created as data moves through a network. Your message starts as application data, but it doesn’t simply jump onto the internet naked. Different networking layers add their own information so devices know what to do with it.
Your Device Starts the Process
Say you’re opening a website. Your browser creates the request first. At this point, there’s no giant packet header sitting around waiting to be attached. The networking software on your device builds the information as the request moves down through the network stack.
And each layer has a job. The transport layer may add information such as source and destination port numbers. Then the network layer adds an IP header, which tells routers where the packet came from and where it’s supposed to go.
The Operating System Does the Heavy Lifting
• Your browser creates the request, but it doesn’t normally build the IP header itself. That job belongs further down the stack.
• TCP or UDP information gets added as the data moves through the transport layer, with the exact fields depending on the protocol being used.
• An IP header appears later, carrying addressing information so routers can make forwarding decisions without needing to understand your actual message.
Headers Get Added Layer by Layer
Think of the data as moving through a series of wrappers. The application creates the useful content first. Then the transport layer wraps it with its own header. The network layer adds another header around that. Finally, the local network technology adds information needed to move the packet across the immediate connection.
This process is called encapsulation. The name sounds more complicated than the idea.
What Happens on the Local Network?
Suppose your laptop sends data through Wi-Fi. Before that data travels across your home network, a Wi-Fi frame needs its own header. That header contains information used by the local network, including addressing information for the immediate connection.
Ethernet does something similar, but with Ethernet frames. So the exact header you see depends on which networking technology is carrying the data at that moment.
Routers Add and Replace Some Headers
Here’s where things get interesting. A router receives a packet and examines its network-layer information. It decides where the packet should go next. As the packet moves between different networks, some surrounding information changes.
The IP header usually travels with the packet from source toward destination, although certain fields can change along the way. The local network header is different. A router removes the old frame information and creates new frame information for the next link.
So Who Actually Creates the Header?
Your device’s networking software creates the headers needed for outgoing traffic. Network hardware and software then handle the parts required for each connection. Routers may update certain fields or create new link-layer headers as packets move onward.
And that’s why packet headers aren’t mysterious information added somewhere out on the internet. They’re built as the data travels through the networking stack.
Once you understand that, packet captures feel much less intimidating. You can look at a header and ask which layer created it and what decision that layer needs to make.