IP Address Classes

IP address classes are part of the classful network architecture that was used before the introduction of CIDR (Classless Inter-Domain Routing). They categorize IP addresses into different classes based on their range and intended usage.

• Class A:
  
  1.0.0.0 to 126.255.255.255
  
  • Designed for very large networks, such as large organizations and enterprises.
  • Private Range: 10.0.0.0 to 10.255.255.255
  • Reserved Addresses: 0.0.0.0/8 (Reserved for default network) and 127.0.0.0/8 (Loopback addresses).
• Class B:
  
  128.0.0.0 to 191.255.255.255
  
  • Suitable for medium-sized networks like schools, universities, and businesses.
  • Private Range: 172.16.0.0 to 172.31.255.255
• Class C:
  
  192.0.0.0 to 223.255.255.255
  
  • Ideal for small networks with fewer hosts, such as small office or home networks (SOHO).
  • Private Range: 192.168.0.0 to 192.168.255.255
• Class D:
  
  224.0.0.0 to 239.255.255.255
  
  • Reserved for multicast groups and cannot be used for regular unicast traffic.
• Class E:
  
  240.0.0.0 to 255.255.255.255
  
  • Reserved for future use, except for the broadcast address 255.255.255.255.

CIDR Notation

Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses and routing IP packets. It provides a more flexible and efficient way to manage IP address allocation compared to the classful system.

Understanding CIDR Notation

CIDR notation consists of an IP address followed by a forward slash (/) and a number indicating the prefix length. The prefix length specifies how many bits are used for the network portion of the address.

Format: IP_Address/Prefix_Length

Example: 192.168.1.0/24

CIDR Calculation for IPv4

To calculate the number of usable IP addresses in an IPv4 CIDR block, use the following formula:

Number of Hosts = 2^{(32 - Prefix_Length)} - 2

In IPv4 subnetting, the first IP address in a subnet is reserved as the network address, which identifies the subnet itself. The last IP address is reserved as the broadcast address, used to send messages to all hosts within the subnet. Consequently, these two addresses cannot be assigned to individual devices, reducing the total number of usable IPs by two.

Example

CIDR Block: 192.168.1.0/24

1. Calculate Number of Hosts:
   2^{(32 - 24)} - 2 = 256 - 2 = 254 usable hosts.
2. IP Range:
   Network Address: 192.168.1.0
   First Usable IP: 192.168.1.1
   Last Usable IP: 192.168.1.254
   Broadcast Address: 192.168.1.255

IPv4 CIDR Range Examples

CIDR Notation	Subnet Mask	Total IPs	Usable IPs	IP Range
/0	N/A	4,294,967,296	4,294,967,294	0.0.0.0 - 255.255.255.255
/8	255.0.0.0	16,777,216	16,777,214	10.0.0.0 - 10.255.255.255
/16	255.255.0.0	65,536	65,534	172.16.0.0 - 172.16.255.255
/20	255.255.240.0	4,096	4,094	172.16.0.0 - 172.16.15.255
/24	255.255.255.0	256	254	192.168.1.0 - 192.168.1.255
/28	255.255.255.240	16	14	192.0.2.0 - 192.0.2.15
/30	255.255.255.252	4	2	192.168.1.0 - 192.168.1.3
/32	255.255.255.255	1	1	203.0.113.5

CIDR Calculation for IPv6

IPv6 uses a fixed /64 prefix length for most subnets, providing 2⁶⁴ addresses per subnet. Unlike IPv4, IPv6 does not subtract addresses for network or broadcast purposes.

Example

CIDR Block: 2001:0db8:85a3::/64

1. Number of Addresses:
   2⁶⁴ = 18,446,744,073,709,551,616 addresses.
2. IP Range:
   Network Prefix: 2001:0db8:85a3::
   First Usable IP: 2001:0db8:85a3::1
   Last Usable IP: 2001:0db8:85a3::ffff:ffff:ffff:ffff

IPv6 CIDR Range Examples

CIDR Notation	Prefix Length	Total IPs	Usable IPs	IP Range
/0	0	2128	2128	:: - ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff
/32	32	296	296	2001:0db8:: - 2001:0db8:ffff:ffff:ffff:ffff:ffff:ffff
/48	48	280	280	2001:0db8:1234:: - 2001:0db8:1234:ffff:ffff:ffff:ffff:ffff
/64	64	264	264	2001:0db8:85a3:: - 2001:0db8:85a3::ffff:ffff:ffff:ffff
/128	128	1	1	2001:0db8:0000:0000:0000:ff00:0042:8329

Tools and Resources

• Subnet Calculator - Calculate subnet masks and IP ranges.
• IP Address Guide - Comprehensive CIDR guide and calculator.

Common Mistakes

• Misclassifying IP Ranges: Confusing private and public IP ranges can lead to improper network configurations and security vulnerabilities.
• Overlapping CIDR Blocks: Assigning overlapping CIDR blocks can cause routing conflicts and connectivity issues within the network.
• Incorrect Usable IP Calculation: Forgetting to subtract two IPs in IPv4 (network and broadcast addresses) can result in assigning reserved addresses to hosts.
• Using Inappropriate Prefix Lengths: Choosing an unsuitable prefix length for the network size can lead to IP address wastage or insufficient IPs for required hosts.
• Ignoring Reserved Addresses: Assigning IPs from reserved ranges (e.g., multicast or future use ranges) can disrupt network operations.
• Incorrect IPv6 Prefix Lengths: Not using the standard /64 prefix for IPv6 subnets can cause compatibility and functionality issues.
• Neglecting Documentation: Failing to document IP allocations and CIDR blocks can complicate network management and troubleshooting.