Subnet Sizing

Classless Inter-Domain Routing (CIDR) is an IP address allocation method that improves data routing efficiency on the internet.

An IP address has two parts:
    • The network address is a series of numerical digits pointing to the network's unique identifier 
    • The host address is a series of numbers indicating the host or individual device identifier on the network

About CIDR

CIDR Corresponds to number of network bits
    /N  →  number of bits used for the network portion

An IPv4 address has 32 bits total.
    Network bits + Host bits = 32

So:

CIDR  Network bits  Host bits
/22 22  10
/28 28  4

Host bits determine subnet size (THIS IS KEY)

Usable IPs

The number of usable IP addresses is: 2^(host bits)

CIDR    Network bits    Host bits   Total IPs
/22 22  10               1024
/28 28  4                  16


Less CIDR (network bits) = More host bits = larger subnet

/22 --> 10 host bits → 2¹⁰ = 1024 IPs
/28 --> 4 host bits → 2⁴ = 16 IPs

Hence, /22 (Large subnet) > /28 (Small subnet)