IP Addressing (IPv4)

The Network Layer (Layer 3) is responsible for end-to-end delivery across multiple networks. The IP address is the logical identifier that makes this possible.

IPv4 Structure

32-bit address written as four decimal octets: 192.168.1.1

Bits	0-7	8-15	16-23	24-31
Octet	192	168	1	1

Every IP address has two parts:

• Network ID — identifies the network segment
• Host ID — identifies the specific device on that network

The boundary between network and host is determined by the subnet mask (e.g., 255.255.255.0 = 24 network bits, 8 host bits).

Public vs Private IP Addresses

Private IP ranges (defined in RFC 1918, not routable on the internet):

Range	CIDR	Use
10.0.0.0 – 10.255.255.255	/8	Large enterprise networks
172.16.0.0 – 172.31.255.255	/12	Medium networks
192.168.0.0 – 192.168.255.255	/16	Home/Small office (default for most routers)

Public IPs are globally unique and assigned by ISPs. Private IPs are reusable across networks — billions of home routers use 192.168.1.x.

Special Addresses

Address	Purpose
127.0.0.1 (localhost)	Loopback — test the local network stack
0.0.0.0	Default route / “all addresses”
255.255.255.255	Limited broadcast
Network address (host bits all 0)	Identifies the network itself
Broadcast address (host bits all 1)	All devices on the network