OSI Model & TCP/IP Stack
The OSI model provides a 7-layer reference framework; TCP/IP is the practical 4-layer implementation. GATE tests layer responsibilities, protocols at each layer, and encapsulation.
Key Points
- ·OSI: Physical → Data Link → Network → Transport → Session → Presentation → Application
- ·TCP/IP: Network Access → Internet → Transport → Application (4 layers)
- ·Encapsulation: data → segment → packet → frame → bits as it descends layers
- ·Physical: bits, signals, cables, hubs, repeaters
- ·Data Link: framing, MAC addresses, error detection, switches, bridges
- ·Network: IP addressing, routing, packet forwarding, routers
- ·Transport: end-to-end delivery, TCP (reliable) and UDP (unreliable)
- ·Application: HTTP, FTP, SMTP, DNS, DHCP — user-facing protocols
- ·PDU names: bits (Physical), frames (Data Link), packets (Network), segments (Transport)
What is a Computer Network?
Analogy: Sending a letter through the postal system. You write the letter (Application), put it in an envelope with an address (Transport/Network), hand it to a truck (Data Link/Physical). Each step adds its own label. At the destination, labels are peeled off in reverse.
This "layering" is exactly what the OSI and TCP/IP models describe.
The OSI 7-Layer Model
Memory trick (bottom to top): "Please Do Not Throw Sausage Pizza Away" Physical, Data Link, Network, Transport, Session, Presentation, Application
Layer 7 - Application: User-facing protocols: HTTP, FTP, SMTP, DNS, DHCP
Layer 6 - Presentation: Format/encrypt/compress: SSL/TLS, JPEG, ASCII
Layer 5 - Session: Manage sessions: NetBIOS, RPC
Layer 4 - Transport: End-to-end delivery: TCP, UDP
Layer 3 - Network: Logical addressing + routing: IP, ICMP, OSPF
Layer 2 - Data Link: Framing + MAC addresses: Ethernet, Wi-Fi (802.11)
Layer 1 - Physical: Raw bits on wire: cables, hubs, signals
| Layer | PDU Name | Device |
|---|---|---|
| Application | Data | — |
| Transport | Segment | — |
| Network | Packet | Router |
| Data Link | Frame | Switch, Bridge |
| Physical | Bits | Hub, Repeater |
TCP/IP Model — The Real Internet
OSI Layers TCP/IP Layers
Application ──────────┐
Presentation ───────── ├── Application (HTTP, FTP, DNS, SMTP, DHCP)
Session ──────────┘
Transport ──────────── Transport (TCP, UDP)
Network ──────────── Internet (IP, ICMP, ARP)
Data Link ──────────┐
Physical ──────────┘── Network Access (Ethernet, Wi-Fi, Fiber)
Encapsulation — Wrapping at Each Layer
Analogy: Your message goes into an envelope (Transport), that envelope goes into a box (Network), that box goes into a delivery truck container (Data Link), which travels as a physical shipment (Physical).
Sender (top-down):
Application creates DATA
Transport adds TCP header → [TCP HDR | DATA] = SEGMENT
Network adds IP header → [IP HDR | SEGMENT] = PACKET
Data Link adds frame header/trailer → [ETH HDR | PACKET | ETH TRL] = FRAME
Physical transmits as bits on wire
Receiver (bottom-up): strip headers in reverse order
Devices and Their Layers
| Device | Layer | How it works |
|---|---|---|
| Hub | Layer 1 | Broadcasts every bit to ALL ports — dumb! |
| Bridge | Layer 2 | Reads MAC, forwards to correct port (or floods) |
| Switch | Layer 2 | Smart bridge — learns MAC table, selective forwarding |
| Router | Layer 3 | Reads IP, looks up routing table, forwards to next hop |
| Gateway | All | Translates between different protocols/architectures |
IP Addressing (IPv4)
IPv4 address = 32 bits = 4 octets: 192.168.1.100
CIDR notation: 192.168.1.0/24 means:
first 24 bits = network part (192.168.1)
last 8 bits = host part (.0 to .255)
Network address: host part all 0s → 192.168.1.0
Broadcast address: host part all 1s → 192.168.1.255
Usable hosts: 2^(32-24) - 2 = 254 hosts
Private IP ranges (not routed on the internet):
10.0.0.0/8 (16 million addresses)
172.16.0.0/12 (1 million addresses)
192.168.0.0/16 (65,000 addresses)
Subnetting — Dividing a Network
Example: You have 192.168.1.0/24 and need 4 subnets.
Need 4 subnets → borrow 2 bits (2² = 4) → new prefix = /26
Each subnet has 2^(32-26) - 2 = 62 usable hosts
Subnet 1: 192.168.1.0/26 (hosts .1 to .62, broadcast .63)
Subnet 2: 192.168.1.64/26 (hosts .65 to .126, broadcast .127)
Subnet 3: 192.168.1.128/26 (hosts .129 to .190, broadcast .191)
Subnet 4: 192.168.1.192/26 (hosts .193 to .254, broadcast .255)
GATE Quick Formula: Usable hosts = 2^h - 2 where h = host bits = 32 - prefix length
Common Protocols at Each Layer
| Protocol | Layer | Purpose |
|---|---|---|
| HTTP/HTTPS | Application | Web pages |
| FTP | Application | File transfer |
| SMTP | Application | Email sending |
| DNS | Application | Name → IP resolution |
| DHCP | Application | Auto-assign IP addresses |
| TCP | Transport | Reliable, ordered delivery |
| UDP | Transport | Fast, unreliable delivery |
| IP | Network | Logical addressing, routing |
| ICMP | Network | Error messages (ping, traceroute) |
| ARP | Data Link/Network | IP → MAC address mapping |
Quick Check
Q1. Which layer adds the IP address header? Answer: Layer 3 — Network layer. IP header contains source and destination IP addresses.
Q2. Network 10.0.0.0/22. How many usable hosts?
Host bits = 32 - 22 = 10 bits
Usable hosts = 2^10 - 2 = 1024 - 2 = 1022 hosts
Q3. A switch vs a router — what is the key difference? Answer: Switch operates at Layer 2 using MAC addresses for forwarding within the same network. Router operates at Layer 3 using IP addresses to forward between different networks.
Key Formulas
- Usable hosts = 2^h - 2 where h = 32 - prefix length
- Network address = IP AND subnet mask
- Broadcast address = IP OR (NOT subnet mask)
GATE Exam Tips
- ★PDU at each layer: bits → frames → packets → segments → data (bottom to top).
- ★Switches operate at Layer 2 (Data Link); routers at Layer 3 (Network).
- ★GATE frequently tests subnetting — practice CIDR calculations quickly.
- ★Transport layer provides end-to-end delivery; network layer provides hop-by-hop routing.
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