Network Bytes · Episode 8 · CCNA 1.5
What is a packet?
Your streaming movie doesn't arrive in one piece. It arrives as thousands of small packets, each labeled with where it's from, where it's going and which piece it is.
Chopped into small pieces
A movie, a photo or a web page is far too big to send as one lump. So your device splits it into many small pieces, and each piece travels on its own as a packet. On a typical home or office network (Ethernet), a packet can carry up to about 1,500 bytes. That limit is called the MTU, the maximum transmission unit.
A one-hour HD movie is a few gigabytes, so it arrives as millions of packets, one after another.
The header: from, to and piece number
Every packet is like an envelope. On the outside is the header, a label with the delivery details:
- the sender's address, such as
192.168.1.25 - the destination address, such as
203.0.113.7 - a number saying where this piece fits, such as
#42
Inside, like a letter in an envelope, is the payload: the actual slice of your movie, message or web page.
Different roads, same destination
Packets don't have to follow each other. Routers along the way send each one on whatever path looks best at that moment, so two packets from the same movie can take different roads and arrive out of order. Your device reads the piece numbers and puts them back in the right order before you ever see the result.
Why small pieces help
Small packets let many people share the same network fairly: your movie's packets take turns on the wire with your family's video calls and games, instead of one big download blocking everyone else.
They also make mistakes cheap. If one packet gets lost on the way, only that one small piece has to be sent again, not the whole file.
Try it yourself
Open a command prompt (Windows) or Terminal (Mac) and run ping 8.8.8.8. Each line is one small packet going out and a reply coming back. On a Mac, press Ctrl+C to stop; at the end you'll see how many packets were sent, received and lost.
Good to know
- 1,500 bytes is the standard Ethernet MTU; other kinds of links can use different sizes.
- Putting pieces back in order and resending lost ones is TCP's job (the TCP handshake episode). UDP traffic, used by many live calls and games, is not resent.
- The addresses shown in the video (
192.168.1.25and203.0.113.7) are examples only.
For CCNA learners
This lines up with CCNA 200-301 exam topic 1.5, compare TCP to UDP: know that TCP uses sequence numbers to reorder segments and retransmits lost data, while UDP does neither.
Test your knowledge
Quick quiz: What is a packet?
4 multiple-choice questions. Pick one answer for each, then check how you did.
Sources
- Cloudflare Learning Center, "What is a packet?": data is split into packets; headers carry source and destination IPs; packets can take different paths
- Cloudflare Learning Center, "What is MTU?": 1,500 bytes is the usual maximum packet size
- IETF, "RFC 9293: Transmission Control Protocol": sequence numbers put data back in order; lost data is retransmitted
- Cisco, CCNA 200-301 v1.1 exam topics: 1.5 compare TCP to UDP
Video transcript
The movie you're streaming doesn't arrive in one piece. It arrives in thousands of tiny envelopes.
Those envelopes are called packets. Before anything crosses a network, it gets chopped into small pieces, usually no bigger than about 1,500 bytes each.
Each packet carries a label, called a header. It holds the sender's address, the destination address, and a number saying where that piece fits. The data itself rides inside, like a letter in an envelope.
Packets don't even have to take the same road. Some go one way, some another, and they can arrive out of order. Your device uses those numbers to put them back together.
Small pieces share the network fairly, and if one gets lost, only that piece is sent again.