The OSI Model and Network Protocols
Following a request from an application down through the network layers and back, and naming the layer where a network problem lives.
Getting a Packet There
The layer model, and how a packet crosses one link and then the networks between two machines.
The OSI model: the seven network layers from cable to application, and how each wraps the data of the layer above it.
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The physical and data link layers: how bits cross one cable or radio link, and how MAC addresses, switches and ARP deliver a frame to the next device.
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IP addresses and subnets: reading CIDR blocks, telling private addresses from public ones, and dividing a range into networks that do not overlap.
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Routing: how routers choose the next hop for a packet, how ping and traceroute see the path, and how the internet routes between networks.
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Between Two Programs
Turning packets into a reliable, private connection between two programs.
Ports, TCP and UDP: how the transport layer delivers data to the right program, and when to choose a reliable stream or a fast datagram.
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How TCP delivers reliably: the handshake, sequence numbers, flow and congestion control, and what one lost segment costs.
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TLS: how HTTPS encrypts a connection and proves the server's identity, and how to read a certificate error.
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Applications and Debugging
The protocols programs speak over a connection, and finding the layer where a request fails.
DNS: how a domain name becomes an IP address, which record types exist, and why a change takes time to reach everyone.
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HTTP on the wire: what a request and response look like, and what HTTP/2 and HTTP/3 change about the connection under them.
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How WebSocket, server-sent events and WebRTC keep a connection open, who may send on it, and how each one survives a drop.
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A network request end to end: every step of opening a web page, and how to find the layer where a failing request breaks.
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