OSI model – functions, scope and examples

Layer	Name	Primary function	Scope	Typical protocols	Data unit
7	Application	Delivers network services directly to user/system applications	Application-to-application	HTTP, FTP, SMTP, DNS, DHCP, SNMP, NTP, SSH	Messages, files, commands
6	Presentation	Formats data: encoding, encryption, decryption, compression	End-to-end (logical)	TLS, SSL, JPEG, PNG, ASCII, MPEG	Data streams, encrypted data
5	Session	Creates, maintains and terminates sessions between applications	Session-to-session	NetBIOS, RPC, NFS, ASP	Sessions, dialogs
4	Transport	Reliable or unreliable end-to-end delivery, flow and congestion control	End-to-end / host-to-host	TCP, UDP, SCTP, DCCP	Segments (TCP), datagrams (UDP)
3	Network	Logical addressing and routing between networks	End-to-end (via routers)	IP, ICMP, OSPF, BGP, RIP, IS-IS	Packets
2	Data Link	Delivery on the same network segment, MAC addressing	Hop-by-hop / node-to-node	Ethernet, Wi-Fi (802.11), PPP, HDLC, ARP	Frames
1	Physical	Transmission of raw bits over the physical medium	Bit-for-bit / media level	Ethernet PHY, RS-232, USB, fiber, Bluetooth	Bits, electrical signals, light, radio

Key concepts

Concept	Explanation
Hop-by-hop	Communication between two directly connected devices (layers 1–2)
Host-to-host	Communication between two end systems (layer 4)
End-to-end	Logical communication sender → receiver across the whole network (layers 3–7)
Node-to-node	Synonym for hop-by-hop
Application-to-application	Communication between software processes (layer 7)

OSI is the reference used for troubleshooting and concept understanding – walk the model down/up to isolate where a failure lives.