Start with the recap, study the fully worked examples, then use the practice problems to
check your understanding of Packet.
This page combines explanation, solved examples, and follow-up practice so you can move
from recognition to confident problem-solving in CS Thinking.
Concept Recap
A small unit of data transmitted over a network, containing both the data payload (the actual information) and routing information in headers (source address, destination address, sequence number). Large messages are split into many packets, sent independently, and reassembled at the destination.
Sending data over a network is like sending a book by mail — you break it into chapters (packets), label each one with the destination, and reassemble at the other end.
Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.
What to Focus On
Core idea:Packet switching means data is broken into pieces that can travel independently — making networks resilient and efficient.
Common stuck point:Packets from the same message can take different routes and arrive out of order — the receiving device reassembles them.
Sense of Study hint:When learning about packets, think of sending a long letter by cutting it into numbered postcards. Each postcard has the destination address and a sequence number. They might travel different postal routes, but the recipient uses the sequence numbers to reassemble the original letter in order.
Worked Examples
Example 1
easy
What is a data packet and why is data sent in packets rather than as one continuous stream?
Answer
Packets are small data chunks with addressing headers. Splitting data into packets enables sharing of network capacity and efficient error recovery.
First step
1
Step 1: A packet is a small chunk of data with a header containing information like source address, destination address, and sequence number.
Full solution
2
Step 2: Splitting data into packets allows multiple communications to share the same network connection (like cars sharing a road).
3
Step 3: If one packet is lost, only that small piece needs retransmitting — not the entire file. This makes data transfer more efficient and reliable.
Packet switching is the foundation of internet communication. It allows millions of users to share network infrastructure simultaneously, unlike circuit switching (telephone) which dedicates a line to each call.
Example 2
medium
Describe what information a packet header contains and why each piece is needed.
Example 3
medium
A 6800-byte file is sent via 1500-byte payloads. How many packets, and how many bytes does the last carry?
Example 4
hard
Compare circuit switching (phone line) vs packet switching (internet) when a link fails mid-message. Why does packet switching survive?When one link fails, packet switching reroutes remaining packets dynamically; circuit switching would drop the entire call
Example 5
hard
For a 100,000-byte file with 1500-byte payload and 40-byte header per packet, compute number of packets and total bytes on wire.
Example 6
challenge
A video call drops 2% of packets but stays usable. Compare this to a file download losing 2% of packets — why does the file still arrive perfect while the video glitches?
Practice Problems
Try these problems on your own first, then open the solution to compare your method.
Example 1
medium
A 1 MB file is split into 1000 packets. Packet 500 is lost during transmission. Describe what happens next.Packet 500 is lost between router 1 and router 2; receiver detects the gap and requests retransmission
Example 2
hard
Explain why packets from the same file might take different routes across the internet and arrive out of order. How does the receiving computer handle this?Two packets from the same file travel different routes and may arrive out of order; sequence numbers allow correct reassembly
Example 3
easy
A packet has two main parts: the actual data and the ____ that holds routing info.
Example 4
easy
Why are large messages split into packets instead of sent as one big block?
Example 5
easy
Do all packets of one message always take the same route? Yes or no.
Example 6
easy
What field lets the receiver reassemble packets in the right order?
Example 7
easy
Is there a maximum size for a single packet? Yes or no.
Example 8
easy
If a packet is lost, does TCP automatically resend it? Yes or no.
Example 9
easy
Reassembling packets into the original message happens at which end: sender or destination?
Example 10
easy
The actual information a packet carries (not the header) is called the ____.
Example 11
medium
A 10000-byte message is sent in packets with max payload 1500 bytes. How many packets are needed?
Example 12
medium
Packets arrive in order 3, 1, 2. Using sequence numbers, what order does the receiver assemble them in?Packets arrive at receiver in order 3, 1, 2; sequence numbers allow reassembly as 1, 2, 3
Example 13
medium
A 4500-byte file uses 1500-byte payloads. If one of the 3 packets is lost and resent, how many packet transmissions occur in total?
Example 14
medium
Why might packets of one download arrive out of order even with no errors?Packets from the same download travel different routes with different delays, causing out-of-order arrival
Example 15
medium
A header adds 40 bytes per packet. For 5 packets, how many total header bytes are sent?
Example 16
medium
If your video call drops a few packets, the picture glitches but the call continues. Which delivery style is this: reliable resend, or tolerate-and-continue?
Example 17
medium
A message split into 8 packets: 6 take route A, 2 take route B. Does the receiver still reconstruct the original message? Yes or no, and via what?8-packet message: 6 packets take route A, 2 take route B; receiver reassembles all 8 via sequence numbers
Example 18
medium
A 7000-byte message uses 1000-byte payloads. How many packets are needed?
Example 19
medium
Packets arrive numbered 2, 4, 1, 3. In what order does the receiver assemble them?Packets arrive in order 2, 4, 1, 3; after reassembly by sequence number the data is delivered as 1, 2, 3, 4
Example 20
challenge
Explain how packet switching makes a network more resilient than sending each message as one indivisible unit over one fixed path.When router 2 fails, packets reroute through router 1 → router 3, demonstrating packet switching resilience
Example 21
challenge
A 1 MB file (1048576 bytes) is sent with 1024-byte payloads. How many packets, and why is the count exact here?
Example 22
challenge
Why does packet loss not necessarily mean the connection is broken? Describe the mechanism that hides the loss from the user.
Example 23
easy
True or false: every packet from one message must take the same path.
Example 24
easy
A packet contains payload plus what other major part?
Example 25
easy
Are packet headers needed at both sender and receiver? Why or why not?
Example 26
easy
If a packet is corrupted in transit, which header field most easily detects the error?
Example 27
easy
Where is a message reassembled from its packets?
Example 28
medium
A 12,000-byte message uses 1500-byte payloads. How many packets needed?
Example 29
medium
A 5000-byte message uses 1500-byte payloads. How many packets?
Example 30
medium
Header overhead is 50 bytes per packet. For 10 packets, total header bytes sent?
Example 31
medium
Packets arrive: 4, 2, 1, 3. After reassembly, in what order is the data delivered to the app?Packets arrive in order 4, 2, 1, 3; sequence numbers allow the receiver to deliver data as 1, 2, 3, 4
Example 32
medium
In TCP, what does the sender do when a packet's ACK doesn't arrive in time?
Example 33
medium
A video stream prefers low latency over perfect reliability. Which protocol style is typical: TCP retransmit, or UDP best-effort?
Example 34
medium
A router decides on a per-packet basis where to send each. What general routing model is this?Router examines each packet individually and forwards it to the best next hop — packet switching
Example 35
medium
Header is 40 bytes, payload is 1460 bytes. What fraction of the packet is overhead?
Example 36
medium
A 30,000-byte message with 1000-byte payloads is sent. How many packets?
Example 37
medium
Two header fields that the receiver uses for reassembly are the source address and the ____ number.
Example 38
hard
A 1 MB file (1,048,576 bytes) is split into 1500-byte payloads. How many packets?
Example 39
hard
Packets in a 100-packet message arrive with 3 lost. Using TCP, how many additional packets does the sender retransmit (minimum)?3 of 100 packets are lost; receiver sends negative acknowledgements (NAKs) and sender retransmits exactly 3 packets
Example 40
hard
A 50-packet stream sees packet 17 arrive twice. How does the receiver handle the duplicate?Packet 17 is duplicated and arrives via two paths; receiver discards the duplicate based on sequence number
Example 41
hard
A 2 MB file (2,097,152 bytes) split into 1024-byte payloads. How many packets?
Example 42
challenge
Why does shrinking MTU increase total bytes on the wire for the same payload? Quantify with header H per packet.