The Ultimate Networking Cheatsheet

Whether you're a developer, student, or sysadmin, understanding networking fundamentals is essential. This cheatsheet covers everything from IPv4 addressing to HTTP status codes.

IPv4 Cheat Sheet

An IPv4 address is a 32-bit number written as four octets (0–255), separated by dots.

Format:   A.B.C.D
Example:  192.168.1.100

IPv4 Address Classes

| Class | Range | Default Subnet Mask | Usage | |-------|---------------------------|---------------------|-------------------| | A | 1.0.0.0 – 126.255.255.255 | 255.0.0.0 (/8) | Large networks | | B | 128.0.0.0 – 191.255.255.255 | 255.255.0.0 (/16) | Medium networks | | C | 192.0.0.0 – 223.255.255.255 | 255.255.255.0 (/24) | Small networks | | D | 224.0.0.0 – 239.255.255.255 | N/A | Multicast | | E | 240.0.0.0 – 255.255.255.255 | N/A | Reserved |

Private IP Ranges (RFC 1918)

10.0.0.0     – 10.255.255.255   (10.0.0.0/8)
172.16.0.0   – 172.31.255.255   (172.16.0.0/12)
192.168.0.0  – 192.168.255.255  (192.168.0.0/16)

Special Addresses

127.0.0.1        → Loopback (localhost)
0.0.0.0          → Default route / unspecified address
255.255.255.255  → Broadcast (all hosts on local network)
169.254.x.x      → APIPA (link-local, no DHCP found)

OSI Model

The OSI (Open Systems Interconnection) model is a 7-layer conceptual framework for how data travels across a network.

| Layer | # | Name | Protocol Examples | Data Unit | |-------|----|--------------|-------------------------|-----------| | 7 | ↑ | Application | HTTP, FTP, DNS, SMTP | Data | | 6 | ↑ | Presentation | SSL/TLS, JPEG, ASCII | Data | | 5 | ↑ | Session | NetBIOS, RPC | Data | | 4 | ↑ | Transport | TCP, UDP | Segment | | 3 | ↑ | Network | IP, ICMP, ARP | Packet | | 2 | ↑ | Data Link | Ethernet, Wi-Fi (802.11)| Frame | | 1 | ↑ | Physical | Cables, Hubs, Signals | Bits |

Mnemonic (top→down): "All People Seem To Need Data Processing"

How Data Flows

Sender                          Receiver
[App Layer]    →  Encapsulate  →  [App Layer]
[Transport]    →  Segment      →  [Transport]
[Network]      →  Packet       →  [Network]
[Data Link]    →  Frame        →  [Data Link]
[Physical]     →  Bits (wire)  →  [Physical]

TCP vs UDP

Both are Layer 4 (Transport) protocols but serve different purposes.

Quick Comparison

| Feature | TCP | UDP | |-------------------|------------------------------|----------------------------| | Connection | Connection-oriented (3-way handshake) | Connectionless | | Reliability | ✅ Guaranteed delivery | ❌ No guarantee | | Ordering | ✅ In-order delivery | ❌ No ordering | | Error Checking | ✅ Yes (ACK/retransmit) | ⚠️ Checksum only | | Speed | Slower (overhead) | ✅ Faster | | Use Cases | Web, Email, File Transfer | Streaming, Gaming, DNS |

TCP 3-Way Handshake

Client          Server
  |  -- SYN -->   |    (1) Client requests connection
  |  <-- SYN-ACK- |    (2) Server acknowledges
  |  -- ACK -->   |    (3) Client confirms → Connection established

Use TCP For:

• HTTP / HTTPS (web browsing)
• FTP (file transfer)
• SMTP, IMAP (email)
• SSH (secure shell)

Use UDP For:

• DNS lookups (fast queries)
• Video/audio streaming
• Online gaming
• VoIP (Voice over IP)
• DHCP

Subnetting Guide

Subnetting divides a large network into smaller, manageable sub-networks.

CIDR Notation

192.168.1.0/24

/24 → 24 bits for network, 8 bits for hosts

Common Subnet Masks

| CIDR | Subnet Mask | # of Hosts | # of Usable Hosts | |------|-------------------|------------|-------------------| | /8 | 255.0.0.0 | 16,777,216 | 16,777,214 | | /16 | 255.255.0.0 | 65,536 | 65,534 | | /24 | 255.255.255.0 | 256 | 254 | | /25 | 255.255.255.128 | 128 | 126 | | /26 | 255.255.255.192 | 64 | 62 | | /27 | 255.255.255.224 | 32 | 30 | | /28 | 255.255.255.240 | 16 | 14 | | /30 | 255.255.255.252 | 4 | 2 |

Usable hosts = 2^(host bits) - 2 (network address + broadcast address)

Subnetting Example

Network:   192.168.1.0/26

Subnet Mask:     255.255.255.192
Network Address: 192.168.1.0
First Host:      192.168.1.1
Last Host:       192.168.1.62
Broadcast:       192.168.1.63
Total Hosts:     64  (62 usable)

Common Ports

Ports are 16-bit numbers (0–65535) that identify specific services on a host.

Well-Known Ports (0–1023)

Port  Protocol  Service
──────────────────────────────────────────
20    TCP       FTP (Data Transfer)
21    TCP       FTP (Control)
22    TCP       SSH (Secure Shell)
23    TCP       Telnet (unencrypted)
25    TCP       SMTP (Send Email)
53    TCP/UDP   DNS (Domain Name System)
67    UDP       DHCP (Server)
68    UDP       DHCP (Client)
80    TCP       HTTP (Web)
110   TCP       POP3 (Receive Email)
143   TCP       IMAP (Email)
443   TCP       HTTPS (Secure Web)
445   TCP       SMB (Windows File Share)
3306  TCP       MySQL
5432  TCP       PostgreSQL
6379  TCP       Redis
8080  TCP       HTTP Alternate
27017 TCP       MongoDB

Quick Reference

80   → HTTP
443  → HTTPS
22   → SSH
21   → FTP
25   → SMTP
53   → DNS
3306 → MySQL
5432 → PostgreSQL
6379 → Redis

DNS Explained

DNS (Domain Name System) translates human-readable domain names into IP addresses.

DNS Resolution Process

Browser → Recursive Resolver → Root Server → TLD Server → Authoritative Server
                                                                      ↓
Browser ← Recursive Resolver ←────────────────────────── IP Address returned

DNS Record Types

| Record | Purpose | Example | |--------|--------------------------------------|--------------------------------------| | A | Maps domain → IPv4 address | example.com → 93.184.216.34 | | AAAA | Maps domain → IPv6 address | example.com → 2606:2800::1 | | CNAME | Alias (canonical name) | www.example.com → example.com | | MX | Mail server for a domain | @ → mail.example.com (priority 10) | | TXT | Text info (SPF, DKIM, verification) | "v=spf1 include:..." | | NS | Name servers for the domain | ns1.example.com | | PTR | Reverse DNS (IP → domain) | 34.216.184.93.in-addr.arpa | | SOA | Start of Authority (zone info) | Serial, refresh, retry intervals |

Common DNS Commands

# Look up IP for a domain
nslookup example.com
dig example.com
 
# Look up a specific record type
dig example.com MX
dig example.com TXT
 
# Reverse DNS lookup
dig -x 93.184.216.34
 
# Flush DNS cache (Windows)
ipconfig /flushdns
 
# Flush DNS cache (Linux/macOS)
sudo systemd-resolve --flush-caches

DNS Hierarchy

.                          (Root)
├── .com                   (TLD - Top Level Domain)
│   └── example.com        (Second Level Domain)
│       └── www.example.com (Subdomain)
├── .org
└── .net

HTTP Status Codes

HTTP status codes indicate the result of a client's request to the server.

1xx — Informational

100 Continue            → Server received headers, client should proceed
101 Switching Protocols → Server switching to a different protocol

2xx — Success ✅

200 OK                  → Request succeeded
201 Created             → Resource successfully created
204 No Content          → Success, no body returned (e.g., DELETE)
206 Partial Content     → Partial resource returned (range requests)

3xx — Redirection 🔀

301 Moved Permanently   → URL has permanently changed (update bookmarks)
302 Found               → Temporary redirect
304 Not Modified        → Cached version is still valid (ETag/Last-Modified)
307 Temporary Redirect  → Redirect, preserve HTTP method
308 Permanent Redirect  → Redirect, preserve HTTP method (permanent)

4xx — Client Errors ❌

400 Bad Request         → Malformed syntax / invalid request
401 Unauthorized        → Authentication required
403 Forbidden           → Authenticated but not authorized
404 Not Found           → Resource doesn't exist
405 Method Not Allowed  → HTTP method not supported for this endpoint
409 Conflict            → State conflict (e.g., duplicate resource)
410 Gone                → Resource permanently deleted
422 Unprocessable Entity→ Validation errors (common in APIs)
429 Too Many Requests   → Rate limit exceeded

5xx — Server Errors 🔥

500 Internal Server Error → Generic server-side error
501 Not Implemented       → Feature not supported by server
502 Bad Gateway           → Invalid response from upstream server
503 Service Unavailable   → Server down or overloaded
504 Gateway Timeout       → Upstream server timed out

Quick Cheat Card

2xx → Success      (200 OK, 201 Created, 204 No Content)
3xx → Redirect     (301 Permanent, 302 Temporary, 304 Cached)
4xx → Client Error (400 Bad, 401 Unauth, 403 Forbidden, 404 Not Found)
5xx → Server Error (500 Internal, 502 Bad Gateway, 503 Unavailable)

Bonus: Key Networking Concepts

NAT (Network Address Translation)

NAT allows multiple devices on a private network to share a single public IP address.

Private: 192.168.1.x  →  [Router/NAT]  →  Public: 203.0.113.5

DHCP (Dynamic Host Configuration Protocol)

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

DORA Process:
Discover → Offer → Request → Acknowledge

ARP (Address Resolution Protocol)

Resolves IP addresses to MAC addresses on a local network.

"Who has 192.168.1.1? Tell 192.168.1.100"  → Broadcast
"192.168.1.1 is at AA:BB:CC:DD:EE:FF"      → Unicast reply

ICMP (Internet Control Message Protocol)

Used for diagnostics and error reporting. Powers ping and traceroute.

ping google.com           # Test connectivity
traceroute google.com     # Trace network path (Linux/macOS)
tracert google.com        # Trace network path (Windows)