Understanding IP Addresses: Classes, Structures, and Reserved Ranges
IP Address Fundamentals
A computer connected to a network requires an IP address for each connection point. This address allows other computers to locate the device on the network. The combination of the network address and the host address creates a unique identifier for each device. Every computer on a TCP/IP network must have a unique IP address, which operates at Layer 3 of the OSI model. This allows computers to locate each other on the network. In addition to IP addresses, computers also have a unique physical address known as a MAC address, assigned by the network interface card manufacturer and operating at Layer 2.
IP Address Classes (A, B, C, D, and E)
IP addresses are divided into classes to accommodate networks of different sizes. Each 32-bit IP address is split into network and host portions. A bit or sequence of bits at the beginning of each class determines its class.
Class A
Designed for large networks (over 16 million host addresses). The first bit is always 0. The first octet range is 1 to 126 (0 and 127 are reserved).
Class B
Designed for moderate to large networks. The first two bits are always 10. The first octet range is 128 to 191.
Class C
Commonly used for small networks (up to 254 hosts). The first three bits are 110. The first octet range is 192 to 223.
Class D
Used for multicasting, allowing a single data stream to be sent to multiple receivers simultaneously. The first four bits are 1110. The first octet range is 224 to 239.
Class E
Reserved for research by the Internet Engineering Task Force (IETF) and not for public use. The first four bits are 1111. The first octet range is 240 to 255.
Reserved IP Addresses
Network Address
Identifies the network itself. For example, in the network 198.150.11.0, any packet sent to that address will be seen from outside the network with the specified IP address. The network address is never assigned as a host address.
Broadcast Address
Used to send packets to all devices on a network. For example, the broadcast address 198.150.11.255 will be read by all hosts on that network.
Public and Private IP Addresses
Public IP Addresses: Unique addresses assigned to devices connected to the public internet. They are globally standardized and ensure that every device has a unique identifier. Public IP addresses are obtained from Internet Service Providers (ISPs).
Private IP Addresses: Used within private networks that are not directly connected to the internet. These addresses can be used freely within the private network as long as they remain unique within that network. Common private IP address ranges include:
- 10.0.0.0 – 10.255.255.255
- 172.16.0.0 – 172.31.255.255
- 192.168.0.0 – 192.168.255.255
Private IP addresses are not routed on the public internet and are a solution to the scarcity of public IP addresses.
Note: The 127.0.0.0 network is reserved for loopback testing, allowing devices to send packets to themselves.