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CORE SUBJECT REFERENCE

COMPUTER
NETWORKS CHEATSHEET.

A comprehensive reference for computer networks – OSI model, TCP/IP, IP addressing, routing, switching, protocols, and more. All in one place.

REFERENCE v1.0

Computer Networks Quick Reference

Everything you need day‑to‑day – protocols, addressing, routing, and more.

OSI Model (7 Layers)

Layer 7 – Application
  • User interface, network applications
  • HTTP, FTP, SMTP, DNS, Telnet, SSH
Layer 6 – Presentation
  • Data formatting, encryption, compression
  • SSL/TLS, JPEG, MPEG, ASCII
Layer 5 – Session
  • Session management, dialog control
  • NetBIOS, RPC, PPTP
Layer 4 – Transport
  • End‑to‑end communication, flow control
  • TCP, UDP, SCTP
Layer 3 – Network
  • Routing, logical addressing
  • IP, ICMP, ARP, RIP, OSPF, BGP
Layer 2 – Data Link
  • Frame delivery, error detection
  • Ethernet, PPP, MAC, VLAN
Layer 1 – Physical
  • Bit transmission, electrical signals
  • USB, Ethernet cables, fiber optics

OSI Mnemonic

  • All People Seem To Need Data Processing
  • Application → Presentation → Session → Transport → Network → Data Link → Physical

TCP/IP Model (4 Layers)

TCP/IP Layer OSI Equivalent Protocols
Application Application, Presentation, Session HTTP, FTP, SMTP, DNS, DHCP, SSH
Transport Transport TCP, UDP
Internet Network IP, ICMP, ARP
Link Data Link, Physical Ethernet, Wi‑Fi, PPP

IP Addressing

IPv4 Address Classes

Class Range Default Mask Purpose Private Range
A 0.0.0.0 – 127.255.255.255 /8 (255.0.0.0) Large networks 10.0.0.0/8
B 128.0.0.0 – 191.255.255.255 /16 (255.255.0.0) Medium networks 172.16.0.0/12
C 192.0.0.0 – 223.255.255.255 /24 (255.255.255.0) Small networks 192.168.0.0/16
D 224.0.0.0 – 239.255.255.255 – Multicast –
E 240.0.0.0 – 255.255.255.255 – Experimental –

Special IPv4 Addresses

  • 0.0.0.0 – default route, unknown address
  • 127.0.0.1 – localhost (loopback)
  • 255.255.255.255 – local broadcast
  • 169.254.0.0/16 – APIPA (automatic private IP)

Subnetting

// Network address = IP & Subnet Mask
// Broadcast address = Network | ~Subnet Mask
// Usable hosts = 2^(32 - prefix) - 2

// Example: 192.168.1.0/24
Network: 192.168.1.0
Broadcast: 192.168.1.255
Usable hosts: 254 (192.168.1.1 – 192.168.1.254)

// Common masks
/8  = 255.0.0.0      (16,777,214 hosts)
/16 = 255.255.0.0    (65,534 hosts)
/24 = 255.255.255.0  (254 hosts)
/28 = 255.255.255.240 (14 hosts)
/30 = 255.255.255.252 (2 hosts) // point‑to‑point links
/32 = 255.255.255.255 (1 host)  // single host

IPv6

  • 128‑bit addresses (vs 32‑bit IPv4)
  • 8 groups of 4 hexadecimal digits
  • Example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
  • Abbreviation: leading zeros can be omitted, :: for consecutive zeros
  • Loopback: ::1
  • Unspecified: ::

Key Protocols

TCP (Transmission Control Protocol)

  • Connection‑oriented – three‑way handshake
  • Reliable – acknowledgements, retransmissions
  • In‑order delivery – sequence numbers
  • Flow control – sliding window, congestion control
  • Ports – well‑known (0‑1023), registered (1024‑49151), dynamic (49152‑65535)
TCP 3‑Way Handshake
Client                            Server
  |                              |
  |--- SYN (seq=x) ------------> |
  |                              |
  |<- SYN-ACK (seq=y, ack=x+1) --|
  |                              |
  |--- ACK (ack=y+1) ----------> |
  |                              |
  |  Connection Established      |

UDP (User Datagram Protocol)

  • Connectionless – no handshake
  • Unreliable – no acknowledgements
  • Out‑of‑order delivery possible
  • Faster – less overhead
  • Use cases: DNS, DHCP, VoIP, streaming, gaming

Port Numbers (Common)

  • 20/21 – FTP
  • 22 – SSH
  • 23 – Telnet
  • 25 – SMTP
  • 53 – DNS
  • 80 – HTTP
  • 110 – POP3
  • 143 – IMAP
  • 443 – HTTPS
  • 3389 – RDP

ICMP (Internet Control Message Protocol)

  • Used for error reporting and diagnostics
  • Type 0 – Echo Reply (ping response)
  • Type 3 – Destination Unreachable
  • Type 8 – Echo Request (ping)
  • Type 11 – Time Exceeded (TTL expired)

ARP (Address Resolution Protocol)

  • Maps IP addresses to MAC addresses
  • ARP request – broadcast, ARP reply – unicast
  • ARP cache – stores mappings

Routing Protocols

Distance Vector
  • RIP – Routing Information Protocol
  • Hop count metric (max 15)
  • Periodic updates (every 30s)
  • Slow convergence
  • Count‑to‑infinity problem
  • IGRP – Cisco proprietary
Link State
  • OSPF – Open Shortest Path First
  • Cost metric (bandwidth based)
  • Dijkstra algorithm
  • Fast convergence
  • No count‑to‑infinity
  • Areas for scalability
Path Vector
  • BGP – Border Gateway Protocol
  • Used between autonomous systems
  • Path attributes for routing decisions
  • Policy‑based routing
Comparison
  • IGP – Interior (RIP, OSPF, EIGRP)
  • EGP – Exterior (BGP)
  • AS – Autonomous System

Switching

Switching Methods
  • Circuit Switching – dedicated path (telephony)
  • Packet Switching – store‑and‑forward
  • Virtual Circuit – connection‑oriented packet
  • Datagram – connectionless packet
Bridge vs Switch vs Router
  • Bridge – Layer 2, connects networks
  • Switch – Layer 2, multi‑port bridge
  • Router – Layer 3, connects networks
  • Hub – Layer 1, broadcast

Network Topologies

Common Topologies
  • Bus – single cable, all share
  • Star – central switch/hub
  • Ring – circular path
  • Mesh – fully connected
  • Tree – hierarchical
  • Hybrid – combination
Network Types by Size
  • PAN – Personal Area Network (Bluetooth)
  • LAN – Local Area Network (Ethernet, Wi‑Fi)
  • MAN – Metropolitan Area Network
  • WAN – Wide Area Network (Internet)
  • WLAN – Wireless LAN
  • VLAN – Virtual LAN

Error Detection & Correction

Error Detection
  • Parity – simple, detects odd errors
  • Checksum – sum of data, used in UDP/TCP
  • CRC – Cyclic Redundancy Check
  • Hamming Code – detection + correction
Error Correction
  • Hamming Code – single‑bit correction
  • Reed‑Solomon – burst errors
  • ARQ – Automatic Repeat Request
  • FEC – Forward Error Correction

Network Security

Encryption
  • Symmetric – AES, DES, 3DES
  • Asymmetric – RSA, ECC
  • Key exchange – Diffie‑Hellman
  • Hashing – MD5, SHA‑1, SHA‑256
Security Protocols
  • SSL/TLS – secure HTTP (HTTPS)
  • IPSec – VPN (AH, ESP)
  • SSH – secure remote access
  • PGP – email encryption
Firewalls
  • Packet Filtering – IP/port based
  • Stateful – tracks connections
  • Proxy – application level
  • WAF – web application firewall
Common Attacks
  • DDoS – Distributed Denial of Service
  • MITM – Man‑in‑the‑middle
  • Phishing – credential theft
  • SQL Injection – database attack
  • XSS – Cross‑Site Scripting

Network Troubleshooting Tools

Command Line Tools
  • ping – test connectivity
  • tracert / traceroute – path to destination
  • nslookup – DNS lookup
  • dig – DNS query tool
  • ipconfig / ifconfig – IP config
  • netstat – network statistics
  • telnet – connect to port
  • curl – HTTP requests
Advanced Tools
  • Wireshark – packet analysis
  • Nmap – port scanning
  • MTR – My TraceRoute (ping + traceroute)
  • tcpdump – CLI packet capture

NAT & DHCP

NAT (Network Address Translation)
  • Static NAT – one‑to‑one mapping
  • Dynamic NAT – pool of IPs
  • PAT (NAT overload) – multiple to one (ports)
  • Benefits: conserve IPs, security
DHCP (Dynamic Host Configuration Protocol)
  • DORA – Discover, Offer, Request, Acknowledge
  • Leases IP address dynamically
  • Provides: IP, subnet, gateway, DNS
  • Renewal at 50% of lease
📌 Quick Reference
OSI layers: Application, Presentation, Session, Transport, Network, Data Link, Physical
TCP: Reliable, connection‑oriented, uses ports, 3‑way handshake
UDP: Unreliable, connectionless, faster, no handshake
IP classes: A (/8), B (/16), C (/24), D (multicast), E (experimental)
Routing: Distance Vector (RIP), Link State (OSPF), Path Vector (BGP)
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