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Overview

Network protocols define the rules and conventions for communication between devices on a network. They ensure reliable delivery, proper addressing, and standardized interactions across diverse systems. Protocols operate at different layers of the OSI model and can be connection-oriented (TCP) or connectionless (UDP).

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Terminology

Term	Definition
Protocol	A set of rules governing how devices communicate on a network
Connection-Oriented	Communication that establishes a session before data transfer (e.g., TCP)
Connectionless	Communication that sends data without establishing a session (e.g., UDP)
ARP Cache	Local table mapping IP addresses to MAC addresses for quick lookup
DORA	DHCP process: Discover → Offer → Request → Acknowledge
TLS/SSL	Encryption layer added to HTTP to create HTTPS
Autonomous System	A network or group of networks under a single administrative domain (used in BGP)

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Core Concepts

Transport Protocols

Full detail covered in Packets and Frames.

Protocol	Type	Use
TCP (Transmission Control Protocol)	Connection-oriented	Reliable, ordered delivery; uses 3-way handshake
UDP (User Datagram Protocol)	Connectionless	Fast, minimal overhead; suitable for streaming and gaming

Addressing Protocols

ARP (Address Resolution Protocol)

Maps IP addresses to MAC addresses on a local network.

ARP Process

• Maintains an ARP cache for quick lookups
• ARP Request — broadcast asking for the MAC address of a given IP
• ARP Reply — unicast response providing the MAC address

DHCP (Dynamic Host Configuration Protocol)

Automatically assigns IP addresses and network configuration to devices.

DHCP DORA

Step	Name	Description
1	Discover	Client broadcasts to find a DHCP server
2	Offer	Server offers an available IP address
3	Request	Client requests the offered IP
4	Acknowledge	Server confirms the assignment

ICMP (Internet Control Message Protocol)

Used for network diagnostics and error reporting.

• ping — tests connectivity and round-trip time
• traceroute — maps the path packets take across networks

Application Protocols

Web

Full detail covered in HTTP & HTTPS.

Protocol	Port	Description
HTTP	80	Web communication
HTTPS	443	Encrypted web communication via TLS/SSL

TLS Negotiation

DNS

Full detail covered in DNS (Domain Name System).

Translates human-readable domain names to IP addresses.

File Transfer

Protocol	Port	Description
FTP	21	Client-server file transfer; commands: USER PASS RETR STOR LIST
FTPS	990	FTP over TLS
SFTP	22	FTP tunneled over SSH

Remote Access

Protocol	Port	Description
SSH	22	Secure encrypted remote terminal access
RDP	3389	Secure graphical remote desktop access
Telnet	23	Insecure plaintext remote terminal; see Linux Fundamentals

File & Device Sharing

Protocol	Port	Description
SMB	445	File and printer sharing across networks

Email Protocols

Protocol	Port	Role	Key Commands
SMTP	25/465/587	Sending email	HELO/EHLO MAIL FROM DATA .
POP3	110/995	Retrieving email (downloads and removes from server)	USER PASS STAT LIST RETR DELE QUIT
IMAP	143/993	Syncing email (keeps mail on server)	LOGIN SELECT FETCH MOVE COPY LOGOUT

Routing Protocols

Determine paths for data across networks by exchanging topology and reachability information.

Protocol	Description
OSPF (Open Shortest Path First)	Shares full network topology; computes shortest paths
RIP (Routing Information Protocol)	Simple hop-count-based routing; max 15 hops
EIGRP (Enhanced Interior Gateway Routing Protocol)	Cisco proprietary; shares reachability and cost metrics
BGP (Border Gateway Protocol)	Internet-wide routing between autonomous systems

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Related Concepts

• OSI Model
• Packets and Frames
• Ports
• IP Addressing
• LAN Topologies
• DNS (Domain Name System)
• HTTP & HTTPS

Related Techniques

• Packet Analysis
• Packet Sniffing

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References / Images

• ARP Process
• DHCP DORA
• TLS Negotiation
• Networking textbooks, RFCs, and vendor documentation