OSI and TCP/IP Models: the Layers, Their Jobs and Devices · ওএসআই ও টিসিপি/আইপি মডেল: স্তর, কাজ ও ডিভাইস

1. Why networks are layered

Sending a message across the world involves many separate problems: turning bits into signals, moving a frame to the next device, finding a route across many networks, delivering data reliably to the right program, and presenting it in a form the program understands. A layered model gives each problem to one layer. Each layer uses the services of the layer below and offers services to the layer above, so one layer can change (Wi-Fi instead of cable, say) without changing the others.

Two models matter:

• OSI (Open Systems Interconnection), a 7-layer reference model published by ISO (ISO/IEC 7498-1). It is mainly a teaching and design reference.
• TCP/IP, the 4-layer model the Internet is actually built on (described in RFC 1122).

2. The seven OSI layers

Learn them from the bottom (layer 1) with a memory aid: Please Do Not Throw Sausage Pizza Away.

#	Layer	Main job	Data unit (PDU)	Address used	Typical devices	Example protocols
7	Application	Network services for applications	Data (message)	–	Gateway (works across layers; usually listed at layer 7)	HTTP, FTP, SMTP, DNS
6	Presentation	Translation, encryption, compression	Data (message)	–	–	(formats such as JPEG, ASCII; TLS is often placed here or in 5–7)
5	Session	Opening, managing and closing sessions (dialog control)	Data (message)	–	–	RPC, NetBIOS
4	Transport	End-to-end delivery, ports, reliability, flow control	Segment (TCP) / Datagram (UDP)	Port number	–	TCP, UDP
3	Network	Logical addressing and routing between networks	Packet	IP address	Router, layer-3 switch	IP, ICMP
2	Data Link	Framing, MAC addressing, error detection on one link	Frame	MAC address	Switch, bridge, NIC	Ethernet, PPP, Wi-Fi (802.11) MAC
1	Physical	Bits as signals on the medium	Bit	–	Hub, repeater, cable, modem	RS-232, 802.3 physical layers

Two neighbouring facts exams like to ask:

• ARP maps an IP address (layer 3) to a MAC address (layer 2), so it sits between the two, and books disagree on its layer: Forouzan lists it with the network-layer protocols (IP, ICMP, IGMP, ARP), RFC 1122 describes it under the Link layer, and some call it "layer 2.5". In MCQs, "Network layer" is the usual keyed answer when it is an option.
• The Data Link layer has two sublayers (IEEE 802): LLC (Logical Link Control, the upper one) and MAC (Media Access Control, the lower one).
• A hub repeats bits to every port (layer 1), a switch forwards frames by MAC address (layer 2), a router forwards packets by IP address (layer 3).

3. The TCP/IP model and how it maps to OSI

TCP/IP layer	Covers OSI layers	What it does	Protocols
Application	7, 6, 5	Everything an application needs	HTTP, HTTPS, DNS, FTP, SMTP, SSH, DHCP
Transport	4	Process-to-process delivery	TCP (reliable, connection-oriented), UDP (fast, connectionless)
Internet	3	Addressing and routing across networks	IP (IPv4, IPv6), ICMP
Link (Network Access)	2, 1	Delivery on the local link and the physical medium	Ethernet, Wi-Fi, PPP

Some textbooks split the Link layer into Data Link and Physical and teach a 5-layer "hybrid" model. The older US Department of Defense (DoD) names are Process/Application, Host-to-Host (= Transport), Internet and Network Access. All three counts appear in exams, so read the question: "OSI" means 7, "TCP/IP" means 4 (or 5 if the options say so).

4. Encapsulation: what happens to your data

On the way down, each layer adds its own header (the Data Link layer also adds a trailer); on the way up, each layer removes it. This is encapsulation and decapsulation.

Worked example: opening a web page. You type a site name in the browser.

1. Application: DNS turns the name into an IP address, then the browser builds an HTTP (or HTTPS) request.
2. Transport: TCP connects to port 443 (HTTPS) or 80 (HTTP) with a three-way handshake (SYN, SYN-ACK, ACK) and splits the data into segments; the sequence number counts bytes (every byte is numbered), not segments.
3. Network: IP adds the source and destination IP addresses; routers along the path read only this header to forward the packet.
4. Data Link: Ethernet or Wi-Fi adds the MAC address of the next hop (usually your router, the default gateway), found with ARP.
5. Physical: the frame leaves as electrical, optical or radio signals.

At the server the layers run in reverse, each removing its header and passing the rest upward.

5. Common ports worth knowing

Port	Protocol	Transport
20, 21	FTP (data, control)	TCP
22	SSH	TCP
23	Telnet	TCP
25	SMTP	TCP
53	DNS	UDP (and TCP)
67, 68	DHCP (server, client)	UDP
80	HTTP	TCP
110	POP3	TCP
143	IMAP	TCP
443	HTTPS	TCP

6. OSI and TCP/IP compared

Point	OSI	TCP/IP
Layers	7	4
Origin	ISO reference model (1984; revised 1994)	Built from the protocols of ARPANET and the Internet
Use	Teaching and design reference	The model the Internet actually runs on
Session and presentation	Separate layers	Inside the Application layer
Protocol fit	Model came first, protocols later	Protocols came first, model described them