07 — Networking Fundamentals

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The OSI Model

The Open Systems Interconnection (OSI) model is a conceptual framework that describes how data travels from one computer to another across a network in 7 layers.

┌─────────────────────────────────────────────────────────────┐
│  Layer 7 — APPLICATION    HTTP, HTTPS, FTP, SMTP, DNS, SSH  │
│  (what the user sees)     Data: Message                     │
├─────────────────────────────────────────────────────────────┤
│  Layer 6 — PRESENTATION   TLS/SSL, JPEG, MPEG, ASCII        │
│  (format/encrypt/decrypt) Data: Formatted Data              │
├─────────────────────────────────────────────────────────────┤
│  Layer 5 — SESSION        NetBIOS, RPC, PPTP                │
│  (manage connections)     Data: Session                     │
├─────────────────────────────────────────────────────────────┤
│  Layer 4 — TRANSPORT      TCP, UDP                          │
│  (end-to-end delivery)    Data: Segments (TCP) / Datagrams  │
├─────────────────────────────────────────────────────────────┤
│  Layer 3 — NETWORK        IP, ICMP, ARP, routing            │
│  (logical addressing)     Data: Packets                     │
├─────────────────────────────────────────────────────────────┤
│  Layer 2 — DATA LINK      Ethernet, Wi-Fi (802.11), MAC     │
│  (physical addressing)    Data: Frames                      │
├─────────────────────────────────────────────────────────────┤
│  Layer 1 — PHYSICAL       Cables, fiber, radio waves        │
│  (bits on wire)           Data: Bits                        │
└─────────────────────────────────────────────────────────────┘

Mnemonic: "All People Seem To Need Data Processing"

Data Encapsulation

As data moves down the stack, each layer wraps the data with its own header:

graph TD
    A["Application Data\n(HTTP Request)"] -->|"Layer 4 adds TCP header"| B
    B["TCP Segment\n[TCP Header | Data]"] -->|"Layer 3 adds IP header"| C
    C["IP Packet\n[IP Header | TCP | Data]"] -->|"Layer 2 adds Ethernet header+trailer"| D
    D["Ethernet Frame\n[ETH Header | IP | TCP | Data | ETH Trailer]"] -->|"Layer 1 transmits bits"| E
    E["Bits on the wire 010101..."]

TCP vs UDP

Feature	TCP	UDP
Connection	Connection-oriented	Connectionless
Reliability	Guaranteed delivery	No guarantee
Order	In-order delivery	No ordering
Error checking	Yes (retransmit)	Basic checksum only
Speed	Slower (overhead)	Faster
Flow control	Yes	No
Use cases	HTTP, SSH, FTP, email	DNS, video streaming, VoIP, gaming

TCP Three-Way Handshake

sequenceDiagram
    participant C as Client
    participant S as Server

    C->>S: SYN (seq=100)
    Note over C: "I want to connect,\nmy seq starts at 100"

    S->>C: SYN-ACK (seq=300, ack=101)
    Note over S: "OK, my seq starts at 300,\nI acknowledge your 100"

    C->>S: ACK (ack=301)
    Note over C: "Acknowledged your 300"

    Note over C,S: Connection Established ✓

    C->>S: Data Transfer...
    S->>C: Data Transfer...

    C->>S: FIN
    S->>C: ACK
    S->>C: FIN
    C->>S: ACK
    Note over C,S: Connection Closed

IP Addressing

IPv4

An IPv4 address is a 32-bit number written as four octets (0–255):

192  . 168  .  1   .  100
 ↑       ↑      ↑      ↑
 8 bits  8 bits 8 bits 8 bits = 32 bits total

IP Address Classes (Historical)

Class	Range	Default Mask	Hosts	Use
A	1–126	/8 (255.0.0.0)	16M	Large orgs
B	128–191	/16 (255.255.0.0)	65K	Medium orgs
C	192–223	/24 (255.255.255.0)	254	Small orgs
D	224–239	N/A	N/A	Multicast
E	240–255	N/A	N/A	Reserved

Private IP Ranges

These ranges are not routable on the internet — used in internal networks:

Range	CIDR	Description
`10.0.0.0` – `10.255.255.255`	`10.0.0.0/8`	Class A private
`172.16.0.0` – `172.31.255.255`	`172.16.0.0/12`	Class B private
`192.168.0.0` – `192.168.255.255`	`192.168.0.0/16`	Class C private
`127.0.0.0` – `127.255.255.255`	`127.0.0.0/8`	Loopback
`169.254.0.0` – `169.254.255.255`	`169.254.0.0/16`	Link-local (APIPA)

Subnet Masks and CIDR

Subnet Mask

A subnet mask separates the network portion from the host portion of an IP:

IP:      192.168.1.100   = 11000000.10101000.00000001.01100100
Mask:    255.255.255.0   = 11111111.11111111.11111111.00000000
                           ↑ 24 bits network             ↑ 8 bits hosts

Network: 192.168.1.0     (all host bits = 0)
Broadcast: 192.168.1.255 (all host bits = 1)
Hosts:   192.168.1.1 – 192.168.1.254 (254 usable)

CIDR Notation

CIDR (Classless Inter-Domain Routing) expresses the mask as a prefix length:

192.168.1.0/24 = 192.168.1.0 with 24-bit mask (255.255.255.0)
10.0.0.0/8     = 10.0.0.0 with 8-bit mask (255.0.0.0)
172.16.0.0/12  = 172.16.0.0 with 12-bit mask (255.240.0.0)

Common Subnet Reference

CIDR	Mask	Hosts	Use Case
`/8`	255.0.0.0	16,777,214	Very large network
`/16`	255.255.0.0	65,534	Large org/campus
`/24`	255.255.255.0	254	Typical LAN
`/25`	255.255.255.128	126	Split /24 in half
`/26`	255.255.255.192	62	Smaller subnet
`/28`	255.255.255.240	14	Small group
`/30`	255.255.255.252	2	Point-to-point links
`/32`	255.255.255.255	1	Single host

Default Gateway

The default gateway is the router that a device sends traffic to when the destination IP is outside the local subnet.

Device: 192.168.1.100/24
GW:     192.168.1.1

Destination: 192.168.1.50  → Same subnet → Direct (no GW needed)
Destination: 8.8.8.8       → Different subnet → Send to GW 192.168.1.1

Network Topology

graph LR
    subgraph LAN 192.168.1.0/24
        PC1["PC1\n192.168.1.100"]
        PC2["PC2\n192.168.1.101"]
        SWITCH["Switch"]
        PC1 --- SWITCH
        PC2 --- SWITCH
    end

    SWITCH --- ROUTER["Router / GW\n192.168.1.1\n(WAN: 203.0.113.5)"]
    ROUTER --- ISP["ISP\nInternet"]
    ISP --- GOOGLE["8.8.8.8\nGoogle"]

DNS — Domain Name System

DNS translates human-readable domain names to IP addresses.

DNS Resolution Process

sequenceDiagram
    participant C as Client
    participant RC as Recursive Resolver (ISP)
    participant ROOT as Root Name Server
    participant TLD as TLD Server (.com)
    participant AUTH as Authoritative Server (google.com)

    C->>RC: "What's the IP of google.com?"
    RC->>ROOT: "Who handles .com?"
    ROOT-->>RC: "TLD server: 192.5.6.30"
    RC->>TLD: "Who handles google.com?"
    TLD-->>RC: "Auth server: 216.239.32.10"
    RC->>AUTH: "What's the IP of google.com?"
    AUTH-->>RC: "172.217.14.238"
    RC-->>C: "172.217.14.238" (+ cached)
    C->>C: Connect to 172.217.14.238

DNS Record Types

Record	Purpose	Example
A	Hostname → IPv4	`google.com → 142.250.x.x`
AAAA	Hostname → IPv6	`google.com → 2607:f8b0::`
CNAME	Alias to another hostname	`www.example.com → example.com`
MX	Mail server	`example.com → mail.example.com`
TXT	Text data (SPF, DKIM, verify)	SPF records
NS	Name server delegation	`example.com → ns1.example.com`
PTR	Reverse DNS (IP → hostname)	`1.2.3.4 → host.example.com`
SOA	Start of Authority — zone info	Serial, refresh, TTL
SRV	Service location	`_sip._tcp.example.com`

# Query DNS records
nslookup google.com              # A record
nslookup -type=MX gmail.com      # MX records
dig google.com                   # Detailed query
dig google.com MX                # MX records
dig +short google.com            # Short answer
dig -x 8.8.8.8                  # Reverse lookup (PTR)

See Networking Tools →

Important DNS Servers

DNS Server	IP	Provider
`8.8.8.8`	Google Public DNS	Google
`8.8.4.4`	Google Public DNS	Google
`1.1.1.1`	Cloudflare	Cloudflare
`1.0.0.1`	Cloudflare	Cloudflare
`9.9.9.9`	Quad9	IBM/Quad9
`208.67.222.222`	OpenDNS	Cisco

Ports and Protocols

A port is a 16-bit number (0–65535) that identifies a specific service on a host.

Port Ranges

Range	Name	Description
0–1023	Well-known ports	Standard services, need root to bind
1024–49151	Registered ports	Third-party services
49152–65535	Dynamic/Ephemeral	Temporary client ports

Common Ports Reference

Port	Protocol	Service
20/21	TCP	FTP (data/control)
22	TCP	SSH
23	TCP	Telnet (insecure, avoid)
25	TCP	SMTP (email sending)
53	TCP/UDP	DNS
67/68	UDP	DHCP (server/client)
80	TCP	HTTP
110	TCP	POP3 (email retrieval)
123	UDP	NTP
143	TCP	IMAP (email)
161/162	UDP	SNMP
389	TCP	LDAP
443	TCP	HTTPS
445	TCP	SMB/CIFS (file sharing)
465/587	TCP	SMTPS / SMTP submission
514	UDP	Syslog
636	TCP	LDAPS (secure LDAP)
993	TCP	IMAPS
995	TCP	POP3S
1433	TCP	Microsoft SQL Server
1521	TCP	Oracle DB
3306	TCP	MySQL
3389	TCP	RDP (Remote Desktop)
5432	TCP	PostgreSQL
5900	TCP	VNC
6379	TCP	Redis
8080	TCP	HTTP alternate
8443	TCP	HTTPS alternate
27017	TCP	MongoDB

DHCP — Dynamic Host Configuration Protocol

DHCP automatically assigns IP addresses, subnet masks, gateways, and DNS to devices.

sequenceDiagram
    participant C as Client
    participant S as DHCP Server

    C->>S: DISCOVER (broadcast: "anyone give me an IP?")
    S->>C: OFFER (192.168.1.50 for 24h)
    C->>S: REQUEST (broadcast: "I want 192.168.1.50")
    S->>C: ACK (confirmed: 192.168.1.50/24, GW: .1, DNS: 8.8.8.8)

    Note over C: Uses IP for lease duration
    Note over C: Renews at 50% of lease time

DORA: Discover → Offer → Request → Acknowledge

NAT — Network Address Translation

NAT allows multiple private IPs to share one public IP.

Private LAN              Router (NAT)           Internet
192.168.1.100 ──────────▶ Maps to 203.0.113.5:12345 ──────▶ 8.8.8.8:80
192.168.1.101 ──────────▶ Maps to 203.0.113.5:12346 ──────▶ 8.8.8.8:80
192.168.1.102 ──────────▶ Maps to 203.0.113.5:12347 ──────▶ 1.1.1.1:443

Types:

PAT/Masquerade — Many-to-one (most common home/office NAT)
Static NAT — One-to-one mapping
Dynamic NAT — Pool of public IPs

IPv6 Basics

IPv6 uses 128-bit addresses written in hexadecimal:

2001:0db8:85a3:0000:0000:8a2e:0370:7334
↓ Simplified (collapse leading zeros and longest :: run)
2001:db8:85a3::8a2e:370:7334

Address	Meaning
`::1`	Loopback (like 127.0.0.1)
`fe80::/10`	Link-local (like APIPA)
`fc00::/7`	Unique local (like private RFC1918)
`ff00::/8`	Multicast
`2000::/3`	Global unicast (internet-routable)

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07 — Networking Fundamentals

The OSI Model

Data Encapsulation

TCP vs UDP

TCP Three-Way Handshake

IP Addressing

IPv4

IP Address Classes (Historical)

Private IP Ranges

Subnet Masks and CIDR

Subnet Mask

CIDR Notation

Common Subnet Reference

Default Gateway

Network Topology

DNS — Domain Name System

DNS Resolution Process

DNS Record Types

Important DNS Servers

Ports and Protocols

Port Ranges

Common Ports Reference

DHCP — Dynamic Host Configuration Protocol

NAT — Network Address Translation

IPv6 Basics

Related Topics

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07 — Networking Fundamentals

The OSI Model

Data Encapsulation

TCP vs UDP

TCP Three-Way Handshake

IP Addressing

IPv4

IP Address Classes (Historical)

Private IP Ranges

Subnet Masks and CIDR

Subnet Mask

CIDR Notation

Common Subnet Reference

Default Gateway

Network Topology

DNS — Domain Name System

DNS Resolution Process

DNS Record Types

Important DNS Servers

Ports and Protocols

Port Ranges

Common Ports Reference

DHCP — Dynamic Host Configuration Protocol

NAT — Network Address Translation

IPv6 Basics

Related Topics