Network+ OSI Model - 7 Layers

OSI Model - The 7 Layers in Order

Layer 7 Application - Layer 6 Presentation - Layer 5 Session - Layer 4 Transport - Layer 3 Network - Layer 2 Data Link - Layer 1 Physical. Mnemonics: top down "All People Seem To Need Data Processing" - bottom up "Please Do Not Throw Sausage Pizza Away". The OSI model was created to achieve interoperability of diverse vendor devices.

Layer 7 - Application

Explanation: Generates data to be transmitted and processes data that is received. It is the entry door to network services; apps use this layer to get services when they send or receive data over a network. Responsible for interfacing user applications, network management services, remote access, etc. Data unit: data. Examples: HTTP (80) - FTP (21) - SMTP (25) - Telnet (23) - POP3 (110) - IMAP (143) - SNMP (161) - TFTP (69).

Layer 6 - Presentation

Explanation: Gets the data ready for the Application layer by converting, translating, encoding, compressing, and encrypting data. Concerned about the format - apps must use a common format. Main services: data conversion - character code translation - compression - encryption and decryption. Data unit: data. Examples: ASCII - EBCDIC - JPEG - TIFF - MPEG - MIDI.

Layer 5 - Session

Explanation: Provides dialog control by allowing multiple persistent connections from different sources to be properly combined or synchronized. Provides a logical persistent connection between peer hosts (the conversation between applications exchanging information) and creates, monitors, and tears down sessions. Modes: full duplex, half duplex, simplex. Data unit: data. Examples: NFS - SQL - NetBIOS - RPC.

Layer 4 - Transport

Explanation: Handles end-to-end communication between peer hosts, either via TCP for connection-oriented communication or UDP for connectionless communication. One of the busiest layers. TCP is connection-oriented and reliable and uses flags; UDP is connectionless, best effort. Data unit: segment (TCP) / datagram (UDP). Examples: TCP - UDP - TLS - SSL - SPX. Port ranges: well-known 0-1023 - registered 1024-49151 - dynamic/private 49152-65535.

Layer 3 - Network

Explanation: Provides communication between different networks via IP addresses - moves data between two hosts that are not physically connected. Uses logical addresses (IP). IP does not guarantee delivery; it only finds the best delivery route, using routing tables. Device: routers. Data unit: packet. Examples: IP - ICMP - OSPF - RIP - BGP - IPSec - NAT - IGMP.

Layer 2 - Data Link

Explanation: Provides communication within the same network via MAC addresses. Gets packets from the Network layer, transmits frames, detects errors within frames, and moves data to the next physically connected device. Two sublayers: LLC (Logical Link Control - flow control and error control) and MAC (Media Access Control - addressing and channel access control). Devices: switches and bridges. Data unit: frame. Examples: Ethernet (IEEE 802.3) - ARP (resolves IP into MAC) - PPP - L2TP - PPTP.

Layer 1 - Physical

Explanation: Converts bits into electrical signals over copper cables, pulses of light for fiber optic cables, or radio frequency for wireless communications. Receives bits from the Data Link layer. Covers physical topologies. Devices: cables, connectors, wireless access point, hubs, modems, etc. Data unit: bits. Examples: 10BASE-T - 100BASE-TX - 1000BASE-T - 1000BASE-SX - DSL - ISDN - SONET.