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What is networking?

How systems talk to each other-IP, DNS, and more.

Basic building blocks

Networking lets computers communicate. IP addresses identify hosts, ports identify services (80, 443, 22). DNS turns names (example.com) into IP addresses. Protocols (TCP, HTTP) define how data is sent.

The network stack has layers: physical (cables, WiFi), link (Ethernet), network (IP), transport (TCP/UDP), application (HTTP, DNS). Each layer has a job-understanding them helps you troubleshoot.

Basic building blocks

IP addresses

Identify hosts (e.g. 192.168.1.1)

Ports

Identify services (80, 443, 22)

DNS

Names → IP (example.com → 93.184.216.34)

Layers

Physical → Link → Network → Transport → App

📡

The network stack

Each layer has a job: physical (cables), link (Ethernet), network (IP), transport (TCP/UDP), application (HTTP, DNS). Understanding layers helps you troubleshoot and design systems.

Network Communication Layers🌐Application LayerHTTP, HTTPS, DNS, SMTP - What users interact withProtocols: Web browsers, email clients, file transfer📦Transport LayerTCP (reliable), UDP (fast) - Manages data deliveryPorts, segmentation, flow control, error checking🔀Network LayerIP Addresses (IPv4, IPv6) - Routes packets between networksRouting, logical addressing, path determination🔌Link LayerEthernet, WiFi - Physical connection between devicesMAC addresses, frame transmission, error detectionExample: Browser Request FlowHTTPRequestTCPSegmentIPPacketEthernetFrameNetworkTransmission

IP addresses and subnets

IPv4 looks like 192.168.1.1; IPv6 like 2001:0db8::1. Most networks still use IPv4.

Subnets divide networks. A subnet mask (e.g. 255.255.255.0) defines which part of the IP is network vs host. VPCs in the cloud let you create isolated networks with custom IP ranges.

IPv4

32-bit, dotted decimal

192.168.1.1

Still dominant; running out of addresses

IPv6

128-bit, hex groups

2001:0db8:85a3::8a2e:0370:7334

Growing; huge address space

Subnet

Subnet mask (e.g. 255.255.255.0) splits an IP into network and host parts. VPCs use custom IP ranges and subnets for isolation.

Network+Host

DNS and name resolution

DNS turns names (example.com) into IPs (93.184.216.34). When you type a URL, your machine queries DNS servers.

DNS is hierarchical: root → TLDs (.com) → domains (example.com) → subdomains (www). Cloud DNS (Route 53, Azure DNS, Cloud DNS) handles resolution, health checks, and routing.

DNS Resolution Flow

1
example.comLocal DNS cache
2
If not cachedRoot DNS servers
3
Root → .comTLD servers
4
.com → example.comAuthoritative servers
5
93.184.216.34IP address returned

TCP, UDP, and protocols

TCP is reliable but slower-guarantees delivery and order. Ideal for web, file transfer, email. UDP is fast but unreliable-ideal for video and gaming where speed beats perfect delivery.

HTTP runs on TCP; HTTPS adds TLS encryption. Understanding these protocols helps you work with any network service.

TCP (Reliable)

Guarantees delivery and order

  • Connection-oriented
  • Error checking
  • Retransmission
  • Slower (overhead)

Use for: Web pages, email, file transfer

UDP (Fast)

No guarantees, but very fast

  • Connectionless
  • Low latency
  • Minimal overhead
  • No delivery guarantee

Use for: Video streaming, gaming, DNS

Layer 4 vs. Layer 7

Layer 4 (transport) routes by IP + port-fast, no content inspection. Layer 7 (application) routes by URL path, hostname, HTTP headers-enables SSL termination and content-based routing.

Load balancers and reverse proxies can operate at either layer. L4 = raw performance; L7 = smarter routing and security.

Layer 4 (Transport)

Fast, handles TCP/UDP

  • Routes by IP + Port
  • Doesn't inspect content
  • Lower latency
  • Good for raw performance

Layer 7 (Application)

Smart, handles HTTP/HTTPS

  • Routes by URL path
  • Can read HTTP headers
  • SSL termination
  • Content-based routing

Load balancers and traffic distribution

Load balancers spread traffic across many servers-better performance and availability (if one fails, others take over). They can work at layer 4 (IP + port) or layer 7 (URL, hostname, headers).

L7 load balancers do SSL termination and content-based routing. L4 is lower latency. Cloud managed load balancers handle scaling, health checks, and SSL.

What load balancers do

Spread the load

Each request goes to one of many servers so no single node melts.

Health checks

Probes backends; stops sending traffic to failed instances.

Algorithms

Round robin, least connections, IP hash—pick the best next server.

Firewalls and network security

Firewalls allow or deny traffic by rules: source/dest IP, port, protocol. Deny by default; allow only what you need (least privilege).

NSGs (network security groups) are cloud firewalls-stateful, attach to VMs or subnets. Essential for securing apps and debugging connectivity.

Allow

Source IP, port, protocol match

Deny

Block everything else by default

In the cloud

NSGs

Cloud firewalls: stateful, attach to VMs/subnets

Least privilege

Only allow traffic you need

How it's used in applications

Apps open connections, send requests, get responses. They use DNS to find servers and TLS/HTTPS for secure channels.

In the cloud, services talk over networks. Networking knowledge helps you design architectures, configure security, and troubleshoot-whether a simple web app or microservices.

How apps use networking

DNS

Find servers by name (example.com → IP)

TLS / HTTPS

Encrypt data in transit

Distributed systems

Services talk over the network

Connectivity

Debug with ping, curl, traceroute

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