Subnetting Reference

Subnetting is the process of dividing a larger network into smaller, more manageable sub-networks, i.e. subnets.

Why subnet?

1. >Efficient IP address management: helps manage IP addresses more efficiently by dividing large networks into smaller segments.
2. >Improved performance: reduces network traffic and congestion by limiting the broadcast domains.
3. >Enhanced security: isolates different segments of a network, improving security and management.

IP address

A unique identifier for a device on a network, e.g. 192.168.1.1.

Subnet mask

A mask used to determine the subnet an IP address belongs to, e.g. 255.255.255.0.

CIDR

Classless Inter-Domain Routing notation is a way to specify IP addresses and their associated routing prefix. It is written as an IP address, followed by a forward slash, and then a number (e.g. 192.168.1.0/24).

• >The number after the slash (e.g. 24 in /24) represents the number of bits in the network prefix.
• >The remaining bits (32 minus the prefix length) represent the host portion.

Some common CIDR notations

• >/24: 255.255.255.0
• >/20: 255.255.240.0
• >/16: 255.255.0.0
• >/8: 255.0.0.0
• >/32: 255.255.255.255 (single IP address)
• >/12: 255.240.0.0

Calculating subnets and hosts

1. >Subnets: divide the available host addresses into smaller groups.
2. >Hosts: determine the number of available IP addresses in each subnet.

Formula for number of hosts

Number of hosts = 2^(32 - prefix length) - 2

The subtraction of 2 accounts for the network and broadcast addresses, which are not assignable to hosts.

Examples

• >/24 (255.255.255.0):
  • >Hosts: 2^(32-24) - 2 = 2^8 - 2 = 254
  • >Subnets: 1 subnet (since /24 is the default subnet mask for Class C)
• >/20 (255.255.240.0):
  • >Hosts: 2^(32-20) - 2 = 2^12 - 2 = 4094
  • >Subnets: 2^(24-20) = 2^4 = 16 subnets if starting from a /24 block
• >/16 (255.255.0.0):
  • >Hosts: 2^(32-16) - 2 = 2^16 - 2 = 65,534
  • >Subnets: 2^(24-16) = 2^8 = 256 subnets if starting from a /24 block
• >/8 (255.0.0.0):
  • >Hosts: 2^(32-8) - 2 = 2^24 - 2 = 16,777,214
  • >Subnets: 2^(24-8) = 2^16 = 65,536 subnets if starting from a /24 block
• >/32 (255.255.255.255):
  • >Hosts: 2^(32-32) - 2 = 2^0 - 2 = 0
  • >Subnets: N/A (single IP address)
• >/12 (255.240.0.0):
  • >Hosts: 2^(32-12) - 2 = 2^20 - 2 = 1,048,574
  • >Subnets: 2^(24-12) = 2^12 = 4,096 subnets if starting from a /24 block

How to remember it

1. >Power of two rule: remember the powers of two (2^8 = 256, 2^10 = 1024, 2^12 = 4096).
2. >Subnet mask patterns:
   • >/24: 255.255.255.0 (256 addresses, 254 usable hosts)
   • >/20: 255.255.240.0 (4096 addresses, 4094 usable hosts)
   • >/16: 255.255.0.0 (65,536 addresses, 65,534 usable hosts)
3. >Bit counting: every bit in the subnet mask reduces the number of host bits by one, and each host bit remaining represents 2 potential addresses.
4. >Shortcut for host counts:
   • >/24: 254 hosts
   • >/23: 512 hosts (minus 2)
   • >/22: 1024 hosts (minus 2)
   • >Continue doubling hosts as we decrease the prefix by 1.

Subnetting reference table