OSI Model
Seven-layer reference model for network communication
L7
Application
HTTP, HTTPS, FTP, SMTP, IMAP, POP3, DNS, DHCP, SNMP
User-facing network services and APIs; closest to the application software.
L6
Presentation
TLS/SSL, JPEG, MPEG, ASCII/EBCDIC conversion
Translation, encryption, and compression so application data can be exchanged reliably.
L5
Session
NetBIOS (session), RPC (session control), SIP (session setup)
Establishes, manages, and tears down dialogs between applications.
L4
Transport
TCP, UDP, SCTP
End-to-end segments, port numbers, reliability (TCP) vs best-effort (UDP).
L3
Network
IPv4, IPv6, ICMP, ICMPv6, OSPF, BGP, IPsec
Logical addressing and routing between networks.
L2
Data Link
Ethernet (802.3), Wi-Fi (802.11), PPP, MAC addresses, switches
Frames on a single link; MAC addressing and LAN switching.
L1
Physical
Copper, fiber, radio; hubs, transceivers, signaling
Bits on the wire or over the air; physical topology and media.
Network+ exam tipNetwork+ tests OSI both top-down and bottom-up. Know which layer each protocol lives on and why. Memorize: L7 Application, L6 Presentation, L5 Session, L4 Transport, L3 Network, L2 Data Link, L1 Physical.