Network Layer & OSI Layers · GATE CSE 2026

Network Layer & Remaining OSI Layers

GATE CSE Revision Notes · 2026

Quick Revision Map

LayerMain ResponsibilityPDU
ApplicationUser ServicesData
PresentationTranslation, EncryptionData
SessionSession ManagementData
TransportEnd‑to‑End DeliverySegments
NetworkRouting & Logical AddressingPackets
Data LinkFraming & MACFrames
PhysicalTransmission of BitsBits

Network Layer (OSI Layer 3)

Functions

  • Logical Addressing (IP Address)
  • Routing
  • Path Determination
  • Packet Forwarding
  • Fragmentation and Reassembly
  • Congestion Control

PDU: Packet

IP Addressing

IPv4

  • 32‑bit Address
  • Divided into 4 Octets
192.168.1.1

Total Addresses: 232

IPv6

  • 128‑bit Address
  • Solves IPv4 exhaustion problem
2001:db8::1

Total Addresses: 2128

Classes of IPv4

ClassFirst Octet RangeDefault Mask
A1–126255.0.0.0
B128–191255.255.0.0
C192–223255.255.255.0
D224–239Multicast
E240–255Experimental

Subnetting

Purpose: Efficient IP utilization, smaller broadcast domains, better security.

Formula

Hosts = 2h − 2
Subnets = 2s

Where h = host bits, s = borrowed bits

Routing

Routing determines the best path from source to destination.

Types

  • Static Routing: Manually configured, simple but less scalable.
  • Dynamic Routing: Automatically updates routes. Examples: RIP, OSPF, BGP.

Routing Algorithms

Distance Vector

  • Example: RIP
  • Uses: Bellman‑Ford Algorithm
  • Metric: Hop Count
  • Maximum: 15 hops

Link State

  • Example: OSPF
  • Uses: Dijkstra's Algorithm
  • Provides faster convergence.

Fragmentation

Occurs when packet size exceeds MTU.

  • IPv4 supports fragmentation by routers.
  • IPv6 routers do not fragment packets.

ICMP

Internet Control Message Protocol — Error Reporting & Diagnostics. Examples: Ping, Destination Unreachable, Time Exceeded.

ARP

Address Resolution Protocol — IP Address → MAC Address

RARP

Reverse ARP — MAC Address → IP Address (obsolete)

DHCP

Dynamic Host Configuration Protocol — automatically assigns IP, Subnet Mask, Gateway, DNS.

NAT

Network Address Translation — conserves public IP addresses, enables private networks to access the Internet.

Private IP Ranges:

  • 10.0.0.0 – 10.255.255.255
  • 172.16.0.0 – 172.31.255.255
  • 192.168.0.0 – 192.168.255.255

Router

Works at Layer 3. Functions: Routing, Packet Forwarding, Broadcast Domain Separation.

Transport Layer (OSI Layer 4)

Functions

  • End‑to‑End Communication
  • Segmentation
  • Flow Control
  • Error Control
  • Multiplexing

PDU: Segment

TCP

Transmission Control Protocol — connection‑oriented, reliable, ordered delivery, error recovery.

Three‑Way Handshake

SYN → SYN+ACK → ACK

UDP

User Datagram Protocol — connectionless, faster, no reliability guarantee. Used in DNS, VoIP, streaming, gaming.

TCP vs UDP

FeatureTCPUDP
ConnectionYesNo
ReliabilityHighLow
SpeedSlowerFaster
OrderingYesNo
AcknowledgmentYesNo

Session Layer (OSI Layer 5)

Session Establishment, Maintenance, Termination. Examples: RPC, NetBIOS.

Presentation Layer (OSI Layer 6)

Data Translation, Encryption, Compression. Examples: JPEG, GIF, MPEG, SSL/TLS.

Application Layer (OSI Layer 7)

Provides services directly to users.

ProtocolPurpose
HTTPWeb Browsing
HTTPSSecure Web
FTPFile Transfer
SMTPEmail Sending
POP3Email Receiving
IMAPEmail Access
DNSName Resolution
TelnetRemote Login
SSHSecure Remote Login

Frequently Asked GATE Facts

TopicRemember
Network LayerLayer 3
Transport LayerLayer 4
RouterLayer 3
IPv432 bits
IPv6128 bits
ARPIP → MAC
DHCPAutomatic IP Allocation
NATConserves Public IPs
TCPReliable
UDPFast
RIPBellman‑Ford
OSPFDijkstra
ICMPPing
HTTPSSecure HTTP

Last‑Minute Revision (2 Minutes)

  • Network Layer = Layer 3
  • PDU = Packet
  • Router works at Layer 3
  • IPv4 = 32 bits
  • IPv6 = 128 bits
  • ARP = IP to MAC
  • DHCP assigns IP automatically
  • NAT saves public IP addresses
  • RIP uses Bellman‑Ford
  • OSPF uses Dijkstra
  • TCP = Reliable, Connection‑Oriented
  • UDP = Fast, Connectionless
  • Session Layer manages sessions
  • Presentation: encryption & compression
  • Application: user services
  • HTTP, HTTPS, FTP, DNS, SMTP = Application Layer

📝 Quick MCQs · Self Check

Click "Reveal Answer" after you attempt each question.

1. Which layer of the OSI model is responsible for routing and logical addressing?

2. What is the size of an IPv4 address?

3. Which protocol is used to map an IP address to a MAC address?

4. Which routing algorithm does OSPF use?

5. Which of the following is a connectionless transport layer protocol?

6. Which layer of the OSI model handles data encryption and compression?

Conclusion

The Network Layer and the remaining OSI layers (Transport, Session, Presentation, Application) form the backbone of modern networking. For GATE CSE, thorough understanding of IP addressing, routing protocols, transport protocols, and application‑layer services is crucial. Practice MCQs and previous‑year questions to solidify these concepts.

Network Layer & OSI Layers · GATE CSE 2026 · Revision ready