Start with a browser request in Brisbane, follow it through switches and routers to a web server in Sydney, watch the sender and receiver TCP/IP stacks work side by side, then construct the complete Ethernet frame one field at a time.
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1
From familiar delivery to network delivery
Send a web request from Brisbane to Sydney
Use the postal analogy to establish the roles of IP addresses, MAC addresses, ports, switches and routers. The analogy is helpful, but the explanation beside every step identifies where the real network behaves differently.
Distinguish end-to-end and local addressingSeparate switch and router decisionsCorrect common postal-analogy limits
The delivery route
Press Next step and follow the highlighted device. The envelope is an analogy for the data being carried, not a literal picture of an Ethernet frame.
Step 1 of 9
✉️
💻
Brisbane laptopBrowser and network stack
🗂️
Local switchReads local MAC labels
🏤
Brisbane routerChooses the next IP route
🌐
Provider networkSeveral routers may forward the packet
🏤
Sydney routerReaches the destination LAN
🗂️
Server switchForwards to the server MAC
🖥️
Sydney web serverHTTPS service on TCP port 443
IP address — the wider destinationLike a street address that helps routers move towards the destination network. In this simplified no-NAT journey, the source and destination IP addresses stay in the packet from sender to receiver.
MAC address — the current local deliveryLike the local label used for the next hand-off on one LAN. Routers replace the Ethernet frame, so the source and destination MAC addresses change at each routed link.
Port number — the service inside the hostLike a department, counter or unit inside the final building. TCP destination port 443 identifies the HTTPS service rather than a physical socket.
Important correction to the analogy. A router does not normally add its own IP address to the packet as a sequence of postal stamps. It examines the destination IP address, reduces the packet’s TTL, chooses a next hop, removes the old link-layer frame and constructs a new frame for the next link. A switch—not a hub—learns local MAC locations.
2
Encapsulation and decapsulation
Build the sender stack and unwrap the receiver stack
The two computers are displayed side by side. At the sender, each lower layer adds information needed for its job. Across the internetwork, routers replace the local Ethernet wrapper. At the receiver, the layers remove and interpret the information in reverse order.
Follow application → transport → Internet → accessSee what routers changeDeliver to the receiving application
One HTTPS request, two stack views
The centre path shows the current network device. The orange token carries a compact summary of what is travelling at the active step.
Step 1 of 13
Sender — Brisbane laptop
ApplicationBrowser creates: GET /lesson-4/
↓
Transport — TCPSource port 51844 · destination port 443
↓
Internet — IP198.51.100.25 → 203.0.113.80
↓
Network access — EthernetLaptop MAC → default-gateway MAC
↓
PhysicalElectrical, light or radio signals carry bits
HTTP dataApplication message
Sender NICCreates and transmits the first frame
Brisbane switchForwards using destination MAC
RoutersForward using destination IP; rebuild frames
Sydney switchForwards to server MAC
Receiver NICChecks and accepts the local frame
Receiver — Sydney web server
PhysicalNIC receives the signal and recovers bits
↑
Network access — EthernetChecks destination MAC and FCS; removes frame
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Internet — IPConfirms 203.0.113.80 and removes IP header
↑
Transport — TCPUses destination port 443 and sequence information
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ApplicationHTTPS web-server software receives the request
The browser creates application dataThe user selects the Lesson 4 page. The browser creates an HTTP request. At this point, the message has not yet been given TCP, IP or Ethernet addressing information.
What stays and what changes?The data begins at the application. No network wrapper has been added yet.
3
Build the frame game
Place every field in the correct nested container
The activity is called “Build the Frame”, but it deliberately shows that IP addresses belong in the IP packet and TCP ports belong in the TCP segment. The packet and segment are carried inside the Ethernet frame rather than being ordinary Ethernet-header fields.
Construct TCP, IP and Ethernet informationExplain the purpose of each fieldObserve hop-by-hop MAC replacement
Build from the inside out
Begin with the browser data. Each press of Add next field reveals the next required item and explains which layer owns it.
0 of 11 fields added
Ethernet frame — one local link14-byte header + payload + 4-byte FCS
Frame check sequenceError-detection value calculated over the frame
Link
Source MAC
Destination MAC
IP packet carried
TTL leaving router
Laptop → Brisbane router
A0-...-25
B0-...-01
198.51.100.25 → 203.0.113.80
64
Brisbane router → provider router
B0-...-02
C0-...-01
198.51.100.25 → 203.0.113.80
63
Provider router → Sydney router
C0-...-02
D0-...-01
198.51.100.25 → 203.0.113.80
62
Sydney router → web server
D0-...-02
E0-...-80
198.51.100.25 → 203.0.113.80
61
Simplification: the table intentionally omits NAT, tunnelling, VLAN tags, IPv6 and many real header fields. It isolates the Week 4 principle: routers preserve the packet’s intended IP destination while replacing the local Ethernet frame for each link.
Ready to begin
Start with the application data
The browser has produced a request, but the networking layers have not yet added their control and addressing information. Press “Add next field” to begin constructing the nested message.
4
Formative review
Check the Week 4 mental model
Answer without returning to the animations. Immediate explanations identify the section worth replaying. The questions focus on device roles and layered reasoning rather than memorising every header field.
Distinguish NIC, switch, router and APExplain encapsulationIdentify hop-by-hop changes