Field notes
IPv4 and IPv6, without the jargon
Two ways to address the same internet. Here is how they differ.
The familiar dotted address
IPv4 uses 32 bits, normally written as four decimal numbers separated by dots, such as 192.0.2.10. Each number is between 0 and 255. Its roughly 4.3 billion possible addresses include ranges reserved for private networks, documentation and other special uses, so not all addresses can be used on the public internet.
A much larger address space
IPv6 uses 128 bits and is written in hexadecimal groups separated by colons, such as 2001:db8::1. Leading zeroes in a group can be omitted, and one consecutive run of zero groups can be compressed to a double colon. This is why the same IPv6 address may have more than one valid written form.
Why both still matter
Many networks run both protocols, called dual stack. A domain can publish an A record for IPv4 and an AAAA record for IPv6. The protocol a device uses depends on its connection, DNS answers and connection behavior. Seeing only IPv4 on a website does not establish whether every part of your network supports IPv6.
Making sense of a prefix
CIDR notation puts a slash and a bit count after an address. An IPv4 /24 contains 256 addresses; an IPv6 /64 contains 2 to the power of 64 addresses. These sizes describe address ranges, not the number of devices actually connected. Use the subnet calculator when you need the exact network boundary or first and last addresses.