IPv4 Addresses and CIDR Notation

An address is 32 bits; the prefix says how many are fixed

An IPv4 address is a 32-bit number written as four decimal octets, such as 10.0.42.7. CIDR notation adds a slash and a prefix length: 10.0.0.0/16 means the first 16 bits are fixed (the network part) and the remaining 16 bits are free (the host part). The block therefore contains 2^(32-16) = 65,536 addresses, from 10.0.0.0 to 10.0.255.255. Each extra bit of prefix halves the block: a /24 has 256 addresses, a /28 only 16. The netmask is the same idea written as an address: /16 is 255.255.0.0. Students often think a larger prefix number means a larger network; it is the opposite, because more bits are locked. Almost every cloud networking decision you make (VPC size, subnet size, route destinations, firewall sources) is written as a CIDR block, so being fluent here saves hours of confusion later.

Exploring a block with Python's ipaddress module

The standard library does the bit arithmetic for you, which is handy for checking plans before you click anything in a console.

import ipaddress

vpc = ipaddress.ip_network("10.0.0.0/16")
print("addresses:", vpc.num_addresses)
print("netmask:  ", vpc.netmask)
print("first/last:", vpc[0], vpc[-1])
print(ipaddress.ip_address("10.0.42.7") in vpc)
print(ipaddress.ip_address("10.1.0.1") in vpc)

Output:

addresses: 65536
netmask:   255.255.0.0
first/last: 10.0.0.0 10.0.255.255
True
False

Quick mental maths

Addresses in a block = 2^(32 - prefix). Memorise a few anchors: /16 = 65,536, /20 = 4,096, /24 = 256, /26 = 64, /28 = 16. Moving one step left or right just doubles or halves the count.

Quick check: How many IPv4 addresses does the block 10.0.0.0/20 contain?

• 4,096
• 1,024
• 65,536
• 256