Understanding IP Address Structure: Classes, Subnetting, and Network Calculations

If an IP address is subnetted, it can be expressed as:

IP address = network address + subnet address + host address

A network address is the logical address of a node on the Internet; MAC addresses operate at the data link layer, IP addresses at the network layer, and port numbers at the transport layer.

Example IP address 192.168.54.4 belongs to a Class C network. Its network address is 192.168.54.0 and its host address is 0.0.0.4.

Each IP address contains two identifiers: a network ID and a host ID. All hosts on the same physical network share the same network ID, while each host has a unique host ID. IP addresses are divided into five classes: A class , B class , C class , D class , and E class .

1. A Class IP Address

An A class address consists of one byte network address and three bytes host address; the highest bit of the network address is 0 . The network range is 1.0.0.0 to 126.0.0.0 with default subnet mask 255.0.0.0. There are 126 usable A networks, each supporting 2^24-2 hosts (about 16 million).

Application: A class addresses are assigned to very large networks such as those of major corporations.

2. B Class IP Address

A B class address uses two bytes for the network address and two bytes for the host address; the highest bits are 10 . The address range is 128.0.0.0 to 191.255.255.255 with default subnet mask 255.255.0.0 . Approximately 2^8 × 64 (≈ 1.6 × 10⁴) B networks exist, each supporting 2^16 hosts (≈ 6 × 10⁴). Application: B class addresses are typically allocated to medium‑sized networks.

3. C Class IP Address

A C class address consists of three bytes network address and one byte host address; the highest bits are 110 . The range is 192.0.0.0 to 223.255.255.255 with default subnet mask 255.255.255.0 . About 2^16 × 32 (≈ 2.09 × 10⁶) C networks exist, each supporting 254 hosts. Application: C class addresses are used for small networks such as typical LANs and campus networks.

4. D Class Address

D class addresses are reserved for multicast; they start with 1110 and do not refer to a specific network.

5. E Class Address

E class addresses start with 11110 and are reserved for future use.

Note: The all‑zero address 0.0.0.0 refers to the current host, and the all‑ones address 255.255.255.255 is the broadcast address of the current subnet.

IP address ranges by class:

A class: 1.0.0.1–126.255.255.254 B class: 128.0.0.1–191.255.255.254 C class: 192.168.0.0–192.168.255.255 D class: 224.0.0.1–239.255.255.254 E class: 240.0.0.1–255.255.255.254 To determine whether two IP addresses belong to the same subnet, perform a bitwise AND between each address and the subnet mask; if the resulting network numbers are identical, the addresses are in the same subnet.

Example using subnet mask 255.255.254.0:

211.95.165.24  11010011.01011111.10100101.00011000
255.255.254.0 11111111.11111111.11111110.00000000
Result:        11010011.01011111.10100100.00000000

211.95.164.78  11010011.01011111.10100100.01001110
255.255.254.0 11111111.11111111.11111110.00000000
Result:        11010011.01011111.10100100.00000000

Both results are identical, confirming the two IPs are in the same subnet.

Default subnet masks without subnetting are: A class: 255.0.0.0 B class: 255.255.0.0 C class: 255.255.255.0 Default (classless): 255.255.255.0