Focused Set (OSI, TCP/IP stack, ports)

OSI Layer 7 - Application

User-facing layer where applications interact with the network — closest to the end user

OSI Layer 6 - Presentation

Handles data formatting, encryption, and compression — translates between application and network formats

OSI Layer 5 - Session

Establishes, maintains, and tears down sessions between systems — sockets live here

OSI Layer 4 - Transport

Provides end-to-end delivery and reliability between hosts

OSI Layer 3 - Network

Logical addressing and routing of traffic between different networks

OSI Layer 2 - Data Link

MAC addressing and frame delivery within a single network segment

OSI Layer 1 - Physical

The actual transmission medium — cables, fiber, radio waves; sends raw bits

Bit

PDU (data unit) at OSI Layer 1 (Physical)

Frame

PDU (data unit) at OSI Layer 2 (Data Link)

Packet

PDU (data unit) at OSI Layer 3 (Network)

Segment

PDU at OSI Layer 4 (Transport) — used by TCP

Datagram

PDU at OSI Layer 4 (Transport) — used by UDP

HTTP (Hypertext Transfer Protocol)

Application layer (OSI 7) — protocol for unencrypted web traffic between browsers and web servers

HTTPS (Hypertext Transfer Protocol Secure)

Application layer (OSI 7) — encrypted web traffic using TLS to protect data in transit

FTP (File Transfer Protocol)

Application layer (OSI 7) — transfers files between client and server in plain text; NOT secure

SSH (Secure Shell)

Application layer (OSI 7) — encrypted remote terminal access; also used for secure file transfer (SFTP/SCP)

Telnet

Application layer (OSI 7) — unencrypted remote terminal access; sends credentials in plain text; NOT secure

SMTP (Simple Mail Transfer Protocol)

Application layer (OSI 7) — sends outgoing email between mail servers

DNS (Domain Name System)

Application layer (OSI 7) — resolves human-readable domain names into IP addresses

POP3 (Post Office Protocol v3)

Application layer (OSI 7) — downloads email from a server to a client and typically removes it from the server

IMAP (Internet Message Access Protocol)

Application layer (OSI 7) — manages email on the server; keeps messages there and syncs across multiple devices

RDP (Remote Desktop Protocol)

Application layer (OSI 7) — Windows remote desktop access protocol

WAF (Web Application Firewall)

Application layer (OSI 7) device — filters HTTP traffic going to a web application

TLS / SSL

Presentation layer (OSI 6) — handles encryption for data in transit

Sockets

Session layer (OSI 5) — an IP address combined with a port number, established during the TCP handshake

NetBIOS (Network Basic Input/Output System)

Session layer (OSI 5) — legacy Windows networking for name resolution, datagrams, and session services; NOT secure

TCP (Transmission Control Protocol)

Transport layer (OSI 4) — connection-oriented with 3-way handshake and guaranteed delivery

UDP (User Datagram Protocol)

Transport layer (OSI 4) — connectionless, best-effort delivery, no handshake, lightweight

IP (Internet Protocol)

Network layer (OSI 3) — logical addressing and routing of packets between networks

ICMP (Internet Control Message Protocol)

Network layer (OSI 3) — used by ping and traceroute for network diagnostics

Router

Network layer (OSI 3) device — connects two or more networks together

Ethernet

Data Link layer (OSI 2) — IEEE 802.3 standard for wired LANs; uses MAC addresses

ARP (Address Resolution Protocol)

Data Link layer (OSI 2) — maps IP addresses to MAC addresses on a LAN

MAC address (Media Access Control)

Data Link layer (OSI 2) — physical hardware address burned into a network interface card

Switch

Data Link layer (OSI 2) device — forwards frames only to the destination port within a LAN

Bridge

Data Link layer (OSI 2) device — connects network segments; older than switches

NIC (Network Interface Card)

Data Link layer (OSI 2) hardware — connects a device to the network

WAP (Wireless Access Point)

Data Link layer (OSI 2) device — provides Wi-Fi connectivity to a network

Hub

Physical layer (OSI 1) device — broadcasts received signal to ALL ports; obsolete, replaced by switches

Modem (Modulator/Demodulator)

Physical/Data Link layer (OSI 1/2) device — converts between digital and analog signals

Cables / Fiber / Radio waves

Physical layer (OSI 1) — the actual transmission medium for signals

TCP/IP Application Layer

Combines OSI layers 5, 6, 7 (Session, Presentation, Application) — handles all user-facing protocols like HTTP, FTP, DNS, SMTP, SSH

TCP/IP Transport Layer

Maps one-to-one with OSI layer 4 — provides end-to-end delivery using TCP (reliable, connection-oriented) and UDP (best-effort, connectionless)

TCP/IP Internet Layer

Maps one-to-one with OSI layer 3 — handles logical addressing and routing using IP and ICMP

TCP/IP Network Access Layer

Combines OSI layers 1 and 2 (Physical and Data Link) — handles physical transmission, MAC addressing, Ethernet, ARP, cables, NICs

Port range 0-1023

Well-known ports — reserved for standard services, assigned by IANA

Port range 1024-49151

Registered ports — vendor-specific applications

Port range 49152-65535

Dynamic / ephemeral ports — spun up by the OS as needed

Total logical ports

65,536 possible values; allowable range is 0 to 65,535 (16-bit number)

Port 21

FTP (File Transfer Protocol) — transfers files in plain text; NOT secure

Port 22

SSH (Secure Shell) — encrypted remote terminal access; also used by SFTP and SCP; SECURE

Port 23

Telnet — unencrypted remote terminal access; sends credentials in plain text; NOT secure

Port 25

SMTP (Simple Mail Transfer Protocol) — sending email in plain text; NOT secure in plain form

Port 53

DNS (Domain Name System) — translates domain names to IP addresses; NOT secure in plain form

Port 80

HTTP (Hypertext Transfer Protocol) — unencrypted web traffic; NOT secure

Port 110

POP3 (Post Office Protocol v3) — downloads email from server to client; NOT secure

Ports 137, 138, 139

NetBIOS — legacy Windows networking; NOT secure

Port 143

IMAP (Internet Message Access Protocol) — manages email on the server; NOT secure

Port 443

HTTPS (HTTP Secure) — encrypted web traffic using TLS; SECURE

Port 3389

RDP (Remote Desktop Protocol) — Windows remote desktop access; security depends on configuration