Framework & FundamentalsFundamentalavailable

Networking Essentials

Master the networking concepts behind system design interviews with layered visuals for requests, transports, and protocol choices.

Flow: layers -> web request -> IP routing -> TCP/UDP/QUIC -> DNS/HTTP/WebSocket/WebRTC -> design choices
Current section
Networking 101
Set the mental model first: application intent rides on top of transport delivery, which rides on top of network routing.
Current step
1 / 3
Layered stack
Interview angle
Explain only the layers that move the request or answer the failure. The strongest answer is the clearest one, not the most encyclopedic one.
Sections

Networking interview agenda

6 guided sections
System Design · Networking

Networking 101

layer-map
The most useful interview shortcut is to map a request through the layers that matter: application, transport, network, and link.
Networking Fundamentals

Networking 101

The most useful interview shortcut is to map a request through the layers that matter: application, transport, network, and link.

1
Core layer

Application Layer

Defines user-facing semantics: names, requests, responses, sessions, and realtime interaction patterns.

Responsibilities
SemanticsHeadersCache hints
Protocols / Concepts
DNSHTTPWebSocketWebRTC signaling
Examples
Page fetchAPI callRealtime control plane
Note:This is where the product contract is expressed.
2
Core layer

Transport Layer

Controls how bytes or datagrams move: reliability, ordering, handshake cost, and congestion behavior.

Responsibilities
DeliveryOrderingLoss recovery
Protocols / Concepts
TCPUDPQUIC
Examples
Reliable API bytesRealtime media packets
Note:This layer determines whether the app sees a stream or datagrams.
3
Core layer

Network Layer

Moves packets between subnets and routers using IP addresses, routing tables, and next-hop decisions.

Responsibilities
AddressingRoutingReachability
Protocols / Concepts
IPv4IPv6ICMP
Examples
Default gatewayVPC subnetInternet routing
Note:IP gives best-effort delivery, not reliability.
4
Core layer

Link / Local Hop

Carries frames across a local segment such as Ethernet or Wi-Fi and enforces the per-link MTU.

Responsibilities
Local deliveryFrame transportMTU boundary
Protocols / Concepts
EthernetWi-Fi
Examples
Home LANData center switch fabric
Note:Most interviews abstract this away, but MTU and local hop constraints explain why the packet shape changes one last time.
application payload -> transport segment/datagram -> IP packet -> link frame
Flow diagram

Networking stack

Layer focus: App -> Transport -> NetworkDebugging lens: where the failure happensInterview goal: pick the right abstraction
Application
user intent
app
HTTP / DNS / WebSocket / WebRTC
Transport
delivery semantics
transport
TCP / UDP / QUIC
Network
packet delivery
network
IP + routing
Link
local hop
link
Ethernet / Wi-Fi
app → transport
active
payload and headers
transport → network
active
segments become packets
network → link
muted
packets become frames
Application
Intent and protocol
HTTP, DNS, WebSocket, and WebRTC define the user-facing contract.
Transport
Delivery semantics
TCP, UDP, and QUIC decide how bytes or datagrams are carried.
Network
Packet routing
IP addresses, subnets, NAT, and routers move traffic between networks.
Why this matters
The application layer defines semantics like HTTP, DNS, and WebSocket.
Interview cue
The transport layer defines delivery behavior such as reliability, ordering, and congestion control.
Interview cue
The network layer moves packets between subnets and routers using IP routing decisions.
Control State
Layer focus
App -> Transport -> Network
Debugging lens
where the failure happens
Interview goal
pick the right abstraction
application payload -> transport segment/datagram -> IP packet -> link frame•Step 1/3