IPv6 notes

The 3 Golden Rules of IPv6 Compression

If you follow these three rules, you will never format an IPv6 address incorrectly:

Rule 1: You can drop leading zeros in any block.

• 0001 becomes 1
• 0db8 becomes db8
• 0000 becomes 0

Rule 2: The :: can replace any contiguous blocks of all zeros.

• fe80:0:0:0:0:0:0:1 becomes fe80::1

Rule 3: You can only use :: ONCE per address.

• This is the most critical rule. If you use it twice, the computer cannot do the math to figure out how many zeros go in each spot.
• Invalid: 2001::abcd::1 (The computer doesn’t know if this means 4 zero blocks then 2 zero blocks, or 2 zero blocks then 4 zero blocks).
• Fix: If you have two separate chunks of zeros, you must use :: on the longest chunk, and use single 0s for the other. Example: 2001:0:0:abcd::1. (If the chunks are equal in length, standard practice is to compress the leftmost chunk).