IP Addressing and Subnetting

IP addressing is the system that assigns a unique identifier to every device on a network. Subnetting is the practice of dividing a large network into smaller, more manageable sub-networks. These concepts are fundamental to network design and administration.

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IPv4 Addressing

An IPv4 address is a 32-bit number, typically written in dotted decimal notation as four octets:

192.168.1.100

Binary:  11000000.10101000.00000001.01100100

Each octet can range from 0 to 255 (8 bits each).

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Network and Host Portions

Every IP address has two parts:

Part	Purpose
Network portion	Identifies which network the device belongs to
Host portion	Identifies the specific device on that network

The subnet mask determines where the network portion ends and the host portion begins.

Subnet Mask Examples

Subnet Mask	CIDR	Network Bits	Host Bits	Usable Hosts
255.0.0.0	/8	8	24	16,777,214
255.255.0.0	/16	16	16	65,534
255.255.255.0	/24	24	8	254
255.255.255.128	/25	25	7	126
255.255.255.192	/26	26	6	62
255.255.255.224	/27	27	5	30
255.255.255.240	/28	28	4	14
255.255.255.252	/30	30	2	2

Formula: Usable hosts = 2^(host bits) - 2 (subtract the network address and broadcast address)

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IP Address Classes (Classful Addressing)

The original classful system divided addresses into classes:

Class	Range	Default Mask	Networks	Hosts per Network
A	1.0.0.0 – 126.255.255.255	/8	126	16,777,214
B	128.0.0.0 – 191.255.255.255	/16	16,384	65,534
C	192.0.0.0 – 223.255.255.255	/24	2,097,152	254
D	224.0.0.0 – 239.255.255.255	—	Multicast	—
E	240.0.0.0 – 255.255.255.255	—	Experimental	—

Note: Classful addressing has been replaced by CIDR (Classless Inter-Domain Routing), which allows subnet masks of any length for more efficient allocation.

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Private vs Public IP Addresses

Type	Purpose	Ranges
Private	Used within internal networks (not routable on the internet)	10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
Public	Globally unique, routable on the internet	Everything else (assigned by ISPs/RIRs)

Devices on a private network access the internet through NAT (Network Address Translation), which maps private addresses to a public address.

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Special IP Addresses

Address	Purpose
127.0.0.1	Loopback (localhost) — always refers to the local machine
0.0.0.0	Default route or "any" address
255.255.255.255	Limited broadcast (local network only)
169.254.x.x	Link-local (APIPA) — auto-assigned when DHCP fails

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