# cidr - Classless Inter-Domain Routing

# Before CIDR and Variable Length Subnet Masks, IP addresses had fixed subnet masks

# Class C had a 24 bit prefix (/24), Class B had a 16 bit prefix (/16), and Class A had an 8 bit prefix (/8)

# Note: The prefix determines how many addresses are covered by the CIDR address. The prefix is the number of bits reserved for the network portion of the address

# An IP address consists of a host and a network portion

# The 32 bit string below represents a /16 network since 16 bits are dedicated to the network portion of the address

# Network bits / Host bits

11111111 11111111 / 00000000 00000000

# As an example, if we were to make a subnet with only 2 addresses on the 3.3.3.0 network, the network prefix would be /31

# If we view this in binary it would look like:

00000011.00000011.00000011.00000000 (3.3.3.0)

# The subnet mask then becomes

11111111.11111111.11111111.11111110 (/31)

# In the subnet mask above, only one bit is available for modification, so the only two available IP addresses would be:

3.3.3.0 and 3.3.3.1

# 3.3.3.2 would represent the start of another subnet

# If we changed the prefix to /30, the new IP address range would be

3.3.3.0 - 3.3.3.3