hafeez@unixshadow:~$ ping -c1 203.0.113.7 # then: what each layer added on the way down
The OSI Model
7 Layers of Networking
every packet you have ever sent walked down this stack — the reply walked back up it.
L7 APPLICATION — user-facing protocols
Where your apps live:
HTTP DNS SMTP FTP SSH — authentication, resource sharing, data formats.travels as: Data
L6 PRESENTATION — translation · encryption · compression
Translates between app and network formats: charsets, serialization, compression.
Encryption lives here —
TLS wraps your traffic before it descends.travels as: Data
L5 SESSION — connections · dialogs
Opens, manages and tears down sessions — who talks to whom, for how long.
Checkpoints and recovery: a dropped transfer resumes instead of restarting.
travels as: Data · e.g.
NetBIOS RPCL4 TRANSPORT — end-to-end delivery
TCP: reliable, ordered, acknowledged — retransmits what gets lost. UDP: fast, fire-and-forget.Ports multiplex everything:
443=HTTPS, 22=SSH, 53=DNS. Segmentation and flow control live here.travels as: Segments (TCP) / Datagrams (UDP)
L3 NETWORK — routing · addressing
IP addressing and routing: how a packet crosses networks it has never seen.Routers live here — so do
ICMP (ping) and routing protocols (OSPF, BGP).travels as: Packets · device: Router
L2 DATA LINK — local delivery
Ethernet + MAC addresses: delivery within one local segment.Switches forward frames by MAC table; VLANs carve one switch into many logical networks.
travels as: Frames · device: Switch
L1 PHYSICAL — the wire itself
Cables, connectors, radio waves — raw bits as electrical, optical or radio signals.
No addresses, no reliability, no mercy: cable unplugged, nothing above matters.
travels as: Bits · device: Hub / cabling
# encapsulation — the packet as it hits the wire:
Each layer wraps the payload from above with its own header (plus a trailer at L2). The receiving host strips them back off in reverse — de-encapsulation on the way up.
# troubleshooting: work bottom-up
L1 first — plugged in? link light on? Then
L1 first — plugged in? link light on? Then
ping (L3), then ports via ss -tlnp / nc -vz (L4), then the app itself (L7). Most “network” problems are physical.# remember the order, top-down
All People Seem To Need Data Processing — Application, Presentation, Session, Transport, Network, Data Link, Physical.
All People Seem To Need Data Processing — Application, Presentation, Session, Transport, Network, Data Link, Physical.
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