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Network Ports & OSI Model Reference

Quick-reference learning guide mapping the standard OSI structural layers with core operational network ports.

Layer 7: Application Layer
Top Layer
How it works: This is the layer closest to the user. It provides network services directly to your software applications (like web browsers or email clients) so you can interact with the network.
Protocols & Port Numbers
HTTP (Port 80) Used for loading standard web pages.
HTTPS (Port 443) Secured, encrypted version of HTTP using certificates.
DNS (Port 53) Translates human-readable names (like yahoo.com) into IP addresses.
FTP (Ports 20, 21) Used for bi-directional file transfers between a client and server.
TFTP (Port 69) Trivial FTP. Faster, connectionless, unauthenticated, uni-directional file transfer.
SMTP (Port 25) Used for sending emails from server to server.
POP3 (Port 110) Used for receiving emails; downloads them locally.
IMAP4 (Port 143) Used for receiving emails; lets you view and manage them directly on the server.
Telnet (Port 23) Basic text-based, unencrypted remote terminal access.
SSH (Port 22) Secure, encrypted remote login and file execution.
SNMP (Ports 161, 162) Used by network managers to monitor device health using "agents" and a MIB database.
SMB (Port 445) Microsoft Windows protocol for transparent file and printer sharing.
LDAP (Port 389) Used to query and modify hierarchical directory services (like phonebooks/user data).
LPD/LPR (Port 515) Network printing systems used primarily for Unix/Linux environments.
NTP (Port 123) Synchronizes the clock/time across network devices.
SCP (Port 22) Securely copies files between hosts using an underlying SSH connection.
Layer 6: Presentation Layer
Data Translation
How it works: Acts as the translator. It takes data from Layer 7 and formats, compresses, or encrypts it so it can be universally understood across different systems.
Protocols & Functions
SSL / TLS Used in HTTPS encryption; lives here to ensure your data is unreadable to eavesdroppers.
Layer 5: Session Layer
Communication Dialogs
How it works: It establishes, manages, maintains, and terminates communication "sessions" (dialogues) between two devices. It keeps your specific connection separate from others.
Protocols & Functions
NetBIOS / RPC This layer coordinates the authentication processes and reconnection management seen in these session-based protocols.
Layer 4: Transport Layer
Segments
How it works: This layer takes data from the upper layers and breaks it into smaller pieces called Segments. It handles how data is delivered between end-to-end applications.
Protocols & Functions
TCP (Transmission Control Protocol) Connection-oriented. It opens a dedicated channel, guarantees data delivery, and verifies receipts. (Used by HTTP, HTTPS, FTP, SMTP).
UDP (User Datagram Protocol) Connectionless. It simply broadcasts data without waiting for verification or acknowledgments, making it much faster. (Used by TFTP, DNS, DHCP).
Layer 3: Network Layer
Packets
How it works: Responsible for routing data across different networks. It takes segments from Layer 4 and packages them into Packets, finding the best path from the source to the destination.
Protocols & Functions
IP (Internet Protocol) Assigns software addresses (IP Addresses) to devices. Includes IPv4 (32-bit) and IPv6 (128-bit).
NAT (Network Address Translation) Translates private internal IP addresses into public IP addresses (Static or Dynamic) to interact with the outside world.
ICMP Used for network diagnostics, error reporting, and router flow control (e.g., the ping command).
IGMP Used to manage and establish memberships in IP multicast groups (online video/gaming).
Layer 2: Data Link Layer
Frames
How it works: Handles physical communication within the same local network domain. It wraps network packets into Frames and ensures error-free physical delivery using physical hardware addresses (MAC Addresses).
Protocols & Functions
ARP (Address Resolution Protocol) Resolves a known IP address (Layer 3) to find a device's unknown physical MAC address (Layer 2).
RARP (Reverse ARP) Used by diskless devices to find their own IP address when they only know their physical MAC address (ancestor of DHCP).
Layer 1: Physical Layer
Bits
How it works: The actual hardware layer. It translates frames into raw electrical, optical, or radio signals (Bits of 1s and 0s) and physically transmits them over cables, fibers, or wireless frequencies.
Physical Components
Hardware Infrastructure Cables (Cat6, Fiber), Hubs, NICs (Network Interface Cards), and connectors.