Network Fundamentals

IP Addressing

Every internet-connected device receives an IP address to identify it on the network.

IPv4 Format

Format: a.b.c.d where each octet is 0–255 (e.g., 192.168.1.100)

Address	Purpose
127.0.0.1	Loopback (localhost)
0.0.0.0	All interfaces / unspecified
255.255.255.255	Broadcast

Public vs Private IP

Type	Description	Example
Public IP	Accessible from internet, globally unique	AWS EC2 public IP, your home router’s WAN IP
Private IP	Inside local network only, not routable on internet	10.0.0.5, 192.168.1.100

Private IP Ranges (RFC 1918)

Range	CIDR	# of Addresses	Common Use
10.0.0.0 – 10.255.255.255	/8	~16.7 million	Large enterprises, cloud VPCs
172.16.0.0 – 172.31.255.255	/12	~1 million	Medium networks
192.168.0.0 – 192.168.255.255	/16	~65,000	Home networks, small offices

Tip

In cloud environments like AWS, you’ll almost always work with private IPs inside your VPC, with public IPs only for internet-facing resources.

IPv6

• 128-bit addresses (vs 32-bit IPv4)
• Format: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
• Expands address space massively; design principles remain similar

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Subnetting & CIDR (Critical for Cloud)

Subnetting divides a network into smaller, manageable segments. CIDR (Classless Inter-Domain Routing) notation makes this precise.

CIDR Notation Explained

10.0.0.0/16
    │     │
    │     └── Network bits (16 bits fixed for network)
    └── Base IP address

CIDR	Network Bits	Host Bits	# of Hosts
/8	8	24	16,777,214
/16	16	16	65,534
/24	24	8	254
/28	28	4	14
/32	32	0	1 (single host)

AWS VPC Example

VPC:        10.0.0.0/16     (65,536 IPs)
            │
            ├── Public Subnet:   10.0.1.0/24   (256 IPs)
            ├── Private Subnet:  10.0.2.0/24   (256 IPs)
            └── Database Subnet: 10.0.3.0/24   (256 IPs)

Caution

AWS reserves 5 IPs per subnet (first 4 + last 1), so a /24 subnet actually gives you 251 usable IPs.

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Ports & Sockets

Ports distinguish different services running on the same host.

Port Ranges

Range	Name	Description
0–1023	Well-known/System	Reserved for standard protocols
1024–49151	Registered	Assigned by IANA for specific services
49152–65535	Dynamic/Ephemeral	Used by clients for temporary connections

Common Ports

Port	Protocol	Service
22	TCP	SSH
53	UDP/TCP	DNS
80	TCP	HTTP
443	TCP	HTTPS
3306	TCP	MySQL
5432	TCP	PostgreSQL
6379	TCP	Redis
27017	TCP	MongoDB

Sockets

A socket is an endpoint for communication: (IP Address, Port)

Client Socket: 192.168.1.100:54321
                    │          │
                    │          └── Ephemeral port
                    └── Client IP

Server Socket: 10.0.1.50:443
                   │      │
                   │      └── Well-known port (HTTPS)
                   └── Server IP

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Transport Layer Protocols

TCP (Transmission Control Protocol)

• Reliable: Guarantees delivery with acknowledgments
• Ordered: Data arrives in sequence
• Connection-oriented: 3-way handshake before data transfer

Client              Server
  │                   │
  │── SYN ──────────▶│
  │◀────── SYN-ACK ──│
  │── ACK ──────────▶│
  │                   │
  │◀═══ Data Flow ═══▶│

Use cases: HTTP/HTTPS, SSH, email, file transfer

UDP (User Datagram Protocol)

• Unreliable: No delivery guarantee
• Unordered: Packets may arrive out of sequence
• Connectionless: No handshake, just send

Use cases: Video streaming, online gaming, DNS queries, VoIP

Comparison

Feature	TCP	UDP
Reliability	Guaranteed	Best-effort
Ordering	Yes	No
Speed	Slower (overhead)	Faster
Use case	Data integrity critical	Real-time, loss-tolerant

TLS/SSL

TLS (Transport Layer Security) sits above TCP to provide:

• Encryption: Data is unreadable to eavesdroppers
• Authentication: Server (and optionally client) identity verified
• Integrity: Data tampering is detected

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DNS (Domain Name System)

DNS translates human-readable domain names to IP addresses.

DNS Resolution Flow

Browser                DNS Resolver           Root/TLD/Auth DNS
   │                        │                        │
   │── "google.com" ───────▶│                        │
   │                        │── Query ──────────────▶│
   │                        │◀── "142.250.185.78" ───│
   │◀── "142.250.185.78" ───│                        │

DNS Record Types

Type	Purpose	Example
A	IPv4 address	google.com → 142.250.185.78
AAAA	IPv6 address	google.com → 2607:f8b0:4004:...
CNAME	Alias to another domain	www.google.com → google.com
MX	Mail server	gmail.com → alt1.gmail-smtp-in.l.google.com
TXT	Text data (SPF, verification)	v=spf1 include:_spf.google.com ~all
NS	Name server	google.com → ns1.google.com

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OSI Model (7 Layers Explained)

The OSI (Open Systems Interconnection) model explains how data travels from your application to another computer across the network. Think of it as 7 steps your data goes through.

How Data Flows

When you send data (e.g., open google.com):

1. Your browser creates the request (Layer 7 - Application)
2. Data gets encrypted with HTTPS (Layer 6 - Presentation)
3. A session/connection is established (Layer 5 - Session)
4. Data is split into segments with port numbers (Layer 4 - Transport)
5. Segments get IP addresses added (Layer 3 - Network)
6. Frames are created with MAC addresses (Layer 2 - Data Link)
7. Converted to electrical/radio signals (Layer 1 - Physical)

When receiving: The process reverses (Layer 1 → 7)

Each Layer Explained

Layer	Name	What It Does	Real Example
7	Application	The actual app/service you use	Chrome browser, email client
6	Presentation	Encrypts, compresses, formats data	HTTPS encryption, JPEG compression
5	Session	Starts, maintains, ends connections	Staying logged into Netflix
4	Transport	Ensures delivery, assigns ports	TCP ensures no data loss, port 443
3	Network	Routes packets across networks	Your IP address, router decisions
2	Data Link	Handles local network delivery	Your laptop’s MAC address, switch
1	Physical	Actual cables and signals	Ethernet cable, WiFi radio waves

Memory Trick

Remember the layers (7 to 1) with: “All People Seem To Need Data Processing”

• Application
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical

Why It Matters for System Design

Layer	System Design Relevance
Layer 7	Application load balancers (ALB), API gateways, WAF
Layer 4	Network load balancers (NLB), faster but less flexible
Layer 3	VPC routing, subnets, security groups

L4 vs L7 Load Balancers

• Layer 4 (Transport): Routes based on IP + Port. Very fast. Can’t see HTTP headers.
• Layer 7 (Application): Routes based on URL path, headers, cookies. Slower but smarter.

Example: L7 can route /api/* to backend servers and /static/* to CDN. L4 cannot.

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TLDR

Concept	Key Points
Public IP	Globally unique, internet accessible
Private IP	Local only, uses RFC 1918 ranges
CIDR	/16 = 65K IPs, /24 = 256 IPs
Ports	0-1023 system, 1024-65535 user
TCP	Reliable, ordered, connection-oriented
UDP	Fast, unreliable, connectionless
DNS	Translates domains to IPs