How do you expand a compressed IPv6 address back to its full form?

Re-pad every hextet to 4 digits, then replace :: with enough 0000 hextets to bring the total back to 8.

Expanding Compressed IPv6 Addresses:

Step 1: Add back leading zeros to each hextet (make each 4 digits)

Step 2: Expand :: to the appropriate number of all-zero hextets

• Count existing hextets
• :: represents (8 - existing count) hextets of 0000

Example: Expand 2001:db8::1

1. Count hextets: 2001, db8, 1 = 3 hextets
2. Missing hextets: 8 - 3 = 5 hextets
3. Replace :: with five 0000 groups

Compressed: 2001:db8::1
Expanded:   2001:0db8:0000:0000:0000:0000:0000:0001

Another Example: fe80::1

• fe80, 1 = 2 hextets present
• :: represents 6 hextets of zeros
• Full: fe80:0000:0000:0000:0000:0000:0000:0001

Go deeper:

• IPv6 address (Wikipedia) — the representation rules read in reverse: re-pad hextets, then size the :: gap to reach 8 groups.