Computer Networks

How the Internet Works

Computer networks allow machines to exchange data. The internet is a global network of networks, bound together by agreed-upon protocols. Understanding networking is essential for building APIs, debugging latency, implementing authentication, and understanding security.

The OSI Model (7 Layers)

The OSI (Open Systems Interconnection) model is a conceptual framework that describes how data moves from one device to another through seven distinct layers. Each layer has a specific responsibility and communicates with the layers directly above and below it.

TCP vs UDP

Both TCP and UDP operate at Layer 4 (Transport). They take data from applications, wrap it with port numbers, and hand it to IP for routing.

TCP 3-Way Handshake

Client                    Server
  |  -- SYN (seq=x) ------>  |   Client: "I want to connect, my seq starts at x"
  |  <-- SYN-ACK (seq=y, ack=x+1) -- |   Server: "OK, my seq starts at y, ack your x"
  |  -- ACK (ack=y+1) ---->  |   Client: "Confirmed"
  |     CONNECTION OPEN      |

// Connection close: FIN → FIN-ACK → FIN → ACK (4-way)

HTTP and HTTPS

HTTP (HyperText Transfer Protocol) is the application-layer protocol of the web. It's request-response: a client sends a request, the server replies. HTTPS wraps HTTP in TLS for encryption.

HTTP Methods

Common Status Codes

DNS — Domain Name System

DNS is the internet's phone book — it translates human-readable hostnames (e.g., google.com) into IP addresses. It's a hierarchical, distributed database.

// DNS resolution for "www.example.com":
1. Browser cache? → use it
2. OS resolver cache (/etc/hosts)? → use it
3. Recursive resolver (ISP or 8.8.8.8):
   a. Ask root nameserver → "I don't know, ask .com TLD server"
   b. Ask .com TLD server  → "I don't know, ask example.com nameserver"
   c. Ask example.com NS   → "www is at 93.184.216.34"
4. Cache result with TTL, return to browser

// Record types:
A     → IPv4 address
AAAA  → IPv6 address
CNAME → Canonical name alias (redirect to another hostname)
MX    → Mail exchange server
TXT   → Arbitrary text (SPF, DKIM, domain verification)

TLS — Transport Layer Security

TLS (what people often call SSL) encrypts data between client and server. HTTPS = HTTP over TLS. TLS ensures: confidentiality (encrypted), integrity (tamper detection), and authentication (server identity via certificate).

// TLS 1.3 handshake (simplified)
Client → Server:  ClientHello (supported cipher suites, key share)
Server → Client:  ServerHello (chosen cipher, key share, certificate)
Client:           Verify certificate against trusted CAs
Client → Server:  Finished (encrypted with shared secret)
Server → Client:  Finished
// Handshake complete — all subsequent data is encrypted

// The shared secret is derived via key exchange (e.g., ECDHE)
// without ever transmitting the secret itself over the wire

IP Addressing

Every device on a network has an IP address — a unique identifier. IPv4 uses 32-bit addresses (e.g., 192.168.1.1); IPv6 uses 128-bit (e.g., 2001:db8::1). CIDR notation (e.g., 192.168.1.0/24) specifies a range — the /24 means 24 bits are the network prefix, leaving 8 bits for hosts (256 addresses).

Private IP ranges (RFC 1918): 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. These are not routable on the public internet — NAT (Network Address Translation) maps them to public IPs.

Key Takeaways

• OSI has 7 layers; in practice, focus on 4 (Transport: TCP/UDP), 3 (Network: IP), and 7 (Application: HTTP/DNS).
• TCP = reliable, ordered, connection-oriented. UDP = fast, unreliable, connectionless.
• HTTP is stateless — cookies and sessions add state on top of it.
• DNS TTL controls how long answers are cached; low TTL means faster propagation, more DNS queries.
• TLS encrypts with symmetric keys exchanged asymmetrically — the certificate proves who you're talking to.