What is a TCP/IP Packet and How Does It Work?

In its simplest form, a packet is the basic unit of information in network transmission. Most networks use TCP/IP as the network protocol, or set of rules for communication between devices, and the rules of TCP/IP require information to be split into packets that contain both a segment of data to be transferred and the address where the data is to be sent.

Because TCP/IP was constructed to use packets, it is considered a packet-switching technology, the primary benefit of which is that data can be routed to a destination through any number of transmission points, making the network decentralized and less vulnerable to equipment failure. As a comparison, networks that use circuit-switching technology like the telephone network must set up a dedicated connection between two points, which has a larger resource footprint and is easier to disrupt.

Every packet that is transmitted over a packet-switching network such as the Internet, the largest such network in existence is constructed of two major pieces: the packet header and the data. Within the header are several distinct pieces of information about the packet itself. This information includes the version of the protocol being used (IPv4 or IPv6), the length of the packet, the number of packets used to send the total data in question, the source and destination addresses, a checksum (used in error correction calculations), and the Time To Live (TTL) data, which defines how many devices the packet may be transmitted along, or hops, before the message is allowed to time out. The data itself is divided into segments of length that can vary, generally in a range of 0 to 64 kB. Packets are transmitted over Ethernet networks, the most common physical type, within frames, or pre-set data blocks that have their own header and trailer information.

Since packets are the basic unit of network transmission, they fit into the standard model of networking model, known as the Open System Interconnection (OSI) model, at the network level, where network devices transmit and configure packet routing, or the paths a given packet will take to reach its destination. After being formed at the network layer, packets are encoded into bits, then passed down to the data link layer. From there, the packets are inserted into frames, and then passed to the physical layer, which is the actual medium of transmission. The process is reversed at the destination, with signal passing to the data link layer, pulling the data as bits from the frame, decoding into packets and passing the packets to the network layer for transmission.

Although the process of packet usage seems complicated, it actually makes the transmission of information far more reliable. Even if two devices are using a protocol that does not guarantee reliable transmission, such as User Datagram Protocol (UDP), a subprotocol of the TCP/IP suite, the fact that each packet includes the source and destination address allows devices to deliver most packets successfully or return failure information back to the source device, allowing for retransmission. Additionally, the header information that describes the number, type and error correction data of the packet allows every packet to provide information about the message as a whole, allowing for easier reconstruction and correction than is possible under circuit-switching transmissions. Short, sturdy and reliable, the structure of packets and packet-switching networks allow for fast and reliable data transmission and make networks like the Internet possible.

How IP Protocols Make Your Life Easy

The details of an IP address and all the technology behind it is important mostly to IT guys, the Internet Technology experts who connect computers at your work or who design the networking hardware and software for your computers.

Thank goodness they have done such a good job that you never have to think about it—because most of us depend on an IP address for everything these days, from getting our news to staying in constant contact with friends and family.

Yet, even though the IP address is part of our everyday, ordinary lives, there is nothing “unordinary” about the way it all comes about. It is all part of a complex and marvelous technological system that runs a worldwide network—the Internet. Here’s one way to understand it all a little better.

IP addresses at work.

If you think about an office building somewhere in your city, you would assume that is has a mailing address. In a network, IPs play a similar role: they help identify where data should be delivered. If you’re not sure how that system is structured, start with what IP addressing is before diving deeper into protocols. And it probably gets mailed delivered to it from advertisers, utility companies, local organizations and so on. It might even get mail from customers or businesses from another state or another country.

Think about that fact.

How does one single piece of mail or a package find its way from across the country to precisely that building, in a matter of days? After all, it’s traveling along with millions of other pieces of mail of all types. Letters, boxes (big and small), postcards and advertising flyers. And it all has to pass through multiple offices, trucks and hands.

And that happens every day, around the clock.

It is all possible thanks to processes, systems, transportation and computers that are intricately linked and working together. The Unites States Post Office, FedEx, and UPS all have in place systems and shipping protocols to make sure it happens quickly and accurately, so that a letter intended for you doesn’t end up somewhere in South America instead.

The post office of the Internet.

In the computer world, your letters, messages and communication (emails, Google searches and more) are also guided by a set of processes or protocols hardwired and programmed right into your computer.

The heart of delivering Internet traffic/data (your activity included) is what’s called TCP/IP technology. The “IP” part stands for “Internet Protocol”—where “IP address” comes from.

TCP/IP technology, the software of the Internet and networking, is designed to work for all us all of the time—no matter what kind of computer we’re using, who our Internet provider is, or what kind of modem or router we have. If you think about it, that’s pretty amazing!

The odds are stacked in your favor.

To make all of the network parts work seamless and perfectly, TCP/IP technology separates network functions into stacks or layers. Most technical books recognize five distinct layers:

The Physical Layer
The Data Link Layer
The Internet Layer
The Transport Layer
The Application Layer

And inside each individual layer are the unique protocols (instructions) for both the networking hardware and software in our computers.

It doesn’t stop there—TCP/IP also has scripted exactly what all those layers should do, as well as when and how it should happen. Those scripts or instructions are the protocols. Taking it one step further, it also defines how those layers should interact with each other.

Sounds complicated. But if you think about it, it all makes sense.

At the post office, just one mailperson isn’t responsible for delivering one piece (or hundreds of pieces) of mail all by themselves. It takes total cooperation from a network of facilities, and types of processes, working simultaneously and in harmony to deliver the mail.

That’s what all of the IP protocols are doing: making sure your Internet requests and email messages get from your lone computer to their correct destinations across town or across the country.