How systems talk to each other-IP, DNS, and more.
How systems talk to each other-IP, DNS, and more.
Lesson outline
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
Identify hosts (e.g. 192.168.1.1)
Identify services (80, 443, 22)
Names → IP (example.com → 93.184.216.34)
Physical → Link → Network → Transport → App
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.
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.
32-bit, dotted decimal
192.168.1.1Still dominant; running out of addresses
128-bit, hex groups
2001:0db8:85a3::8a2e:0370:7334Growing; huge address space
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.
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
example.comLocal DNS cacheIf not cachedRoot DNS serversRoot → .comTLD servers.com → example.comAuthoritative servers93.184.216.34IP address returnedTCP 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.
Guarantees delivery and order
Use for: Web pages, email, file transfer
No guarantees, but very fast
Use for: Video streaming, gaming, DNS
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.
Fast, handles TCP/UDP
Smart, handles HTTP/HTTPS
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
Each request goes to one of many servers so no single node melts.
Probes backends; stops sending traffic to failed instances.
Round robin, least connections, IP hash—pick the best next server.
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.
Source IP, port, protocol match
Block everything else by default
In the cloud
Cloud firewalls: stateful, attach to VMs/subnets
Only allow traffic you need
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
Find servers by name (example.com → IP)
Encrypt data in transit
Services talk over the network
Debug with ping, curl, traceroute
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