Private and Public IP Addresses; OSI Reference Model

Private and Public IP Addresses

Private IP Address Ranges

Private IP addresses are NOT routable on the internet. Used within private networks (home, office).

Class	Private Range	CIDR
Class A	10.0.0.0 – 10.255.255.255	10.0.0.0/8
Class B	172.16.0.0 – 172.31.255.255	172.16.0.0/12
Class C	192.168.0.0 – 192.168.255.255	192.168.0.0/16

Special Addresses:

Address	Purpose
127.0.0.1 (loopback)	Refers to the local machine itself
169.254.x.x (APIPA)	Self-assigned when DHCP fails
0.0.0.0	Non-routable; default route
255.255.255.255	Limited broadcast (current network)

Public IP Addresses

Assigned by ISPs
Globally unique and routable on the internet
All addresses NOT in the private range

NAT (Network Address Translation)

NAT allows multiple devices with private IPs to share one public IP address.

Private Network                    Internet
192.168.1.100 ─┐
192.168.1.101 ─┤──> [NAT Router] ─────> 203.0.113.1 (Public IP) ──> Internet
192.168.1.102 ─┘

OSI Reference Model

What is OSI Model?

The OSI (Open Systems Interconnection) Model is a conceptual framework that standardizes the functions of a communication system into 7 layers.

Developed by ISO (International Standards Organization) in 1984.

Purpose:

Universal reference model for networking
Allows products from different vendors to interoperate
Helps troubleshoot network issues layer by layer

The 7 Layers of OSI Model

+---+-------------------+
| 7 | Application       |  ---> User interface (HTTP, FTP, SMTP)
+---+-------------------+
| 6 | Presentation      |  ---> Data format, encryption, compression
+---+-------------------+
| 5 | Session           |  ---> Session management, synchronization
+---+-------------------+
| 4 | Transport         |  ---> End-to-end communication (TCP, UDP)
+---+-------------------+
| 3 | Network           |  ---> Logical addressing, routing (IP)
+---+-------------------+
| 2 | Data Link         |  ---> Physical addressing, error detection (Ethernet, Wi-Fi)
+---+-------------------+
| 1 | Physical          |  ---> Bits on wire (voltage, cables, connectors)
+---+-------------------+

Mnemonic: "Please Do Not Throw Sausage Pizza Away" (Physical, Data Link, Network, Transport, Session, Presentation, Application)

Layer Details

Layer	Name	Function	PDU	Protocol/Device
7	Application	User interface, application services	Message/Data	HTTP, FTP, SMTP, DNS, DHCP
6	Presentation	Data format, encryption, compression	Message/Data	SSL/TLS, JPEG, MPEG
5	Session	Session management, dialog control	Message/Data	NetBIOS, RPC
4	Transport	End-to-end data transfer, reliability	Segment	TCP, UDP
3	Network	Logical addressing, routing	Packet	IP, ICMP, IPsec — Router
2	Data Link	Physical addressing, error detection	Frame	Ethernet, Wi-Fi (802.11) — Switch, Bridge
1	Physical	Bit transmission over medium	Bit	Cables, hubs, repeaters, connectors

PDU = Protocol Data Unit (unit of data at each layer)

Data Encapsulation

Data flows down through layers (sender) and up through layers (receiver):

Application: [DATA]
Presentation: [DATA]
Session: [DATA]
Transport: [Segment Header][DATA]
Network: [Packet Header][Segment Header][DATA]
Data Link: [Frame Header][Packet Header][Segment Header][DATA][Frame Trailer]
Physical: 010110010100... (bits on wire)

Troubleshooting Using OSI Model

Layer 1: Physical → Check cables, connectors, NICs, power
Layer 2: Data Link → Check MAC address, VLAN, switch config
Layer 3: Network → Check IP address, subnet mask, routing
Layer 4: Transport → Check ports, firewall rules, TCP/UDP
Layer 5-7: Application → Check application, DNS, authentication