Turning Point Academy · Network Essentials

OSI and TCP/IP Model Reference

One-page companion for the whole course. Print it, keep it beside you.

The two models, side by side

OSI #	OSI layer	Parcel (PDU)	Typical residents	TCP/IP layer
7	Application	Data	HTTP/HTTPS, DNS, DHCP, SMTP/IMAP, SSH	Application
6	Presentation	Data	Encodings, compression (in practice: inside apps)	Application
5	Session	Data	Dialogue open/close (in practice: TCP + apps)	Application
4	Transport	Segment (TCP) / Datagram (UDP)	TCP, UDP — ports live here	Transport
3	Network	Packet	IP (v4/v6), ICMP, routers, firewalls	Internet
2	Data Link	Frame	Ethernet, Wi-Fi, ARP, switches, NICs, APs	Network Access
1	Physical	Bits / signals	Cables, hubs, repeaters, radio, connectors	Network Access

Memory aid (bottom → top): Please Do Not Throw Sausage Pizza Away.

Encapsulation at a glance

SEND (wrap, top→down)                    RECEIVE (open, bottom→up)
application data                          read the data
  + TCP/UDP header  = SEGMENT               open segment (port → application)
    + IP header     = PACKET                  open packet (is this my IP?)
      + MAC header  = FRAME (+ trailer)         open frame (is this my MAC?)
        → signals on the medium             ← signals from the medium

IP addresses are end-to-end; MAC addresses are one-hop-only. Outbound frames usually carry your default gateway's MAC, and every router rebuilds the frame while the packet inside rides unchanged.
Headers are small field databases. IPv4's key fields: Version, DSCP (priority; the field once called Type of Service), TTL (hop budget), Protocol (6 = TCP, 17 = UDP, 1 = ICMP), Source IP, Destination IP.

Device / decision map

Device	Decides using	Layer
Hub, repeater (historic)	Nothing — repeats signals	1
Switch, bridge, access point	MAC address	2
Router, traditional firewall	IP address	3
Load balancer / NGFW (app-aware)	Ports and application data	4-7

Layer placements people get wrong

Three-way handshake (SYN, SYN-ACK, ACK): TCP — layer 4. Not a session-layer protocol.
ICMP (ping): rides inside IP as protocol 1 — layer 3. Not transport.
IPsec: protects IP packets — layer 3. Not presentation, despite "encryption".
Your browser: an application. HTTP is the layer-7 protocol it speaks.

Troubleshooting ladder (bottom-up)

Physical — plugged in? link light? interface up?
Data link — on the right network/VLAN? frames flowing?
Network — ping yourself → gateway → far host.
Transport — right port open? service listening?
Application — service configured? name lookup working? app errors?

Cheapest checks first: each passed test certifies every layer below it.

Course: Network Essentials — Turning Point Academy. Academic backbone: Al-Doori, Network Essentials, Chapters 2 and 5. Current TCP spec: RFC 9293.