What expansion puts back
A compressed address leaves out two kinds of zero. Leading zeros inside a group are dropped, so 0db8 is written db8, and one run of all-zero groups is collapsed into a double colon. Expansion restores both. The result is the address as it sits in the packet header: 128 bits, sixteen bytes, each hex digit standing for one four-bit nibble.
Worked through for 2001:db8::ff00:42:8329: five groups are written, so the double colon stands for three groups of 0000. Pad db8 to 0db8 and 42 to 0042, and the address reads 2001:0db8:0000:0000:0000:ff00:0042:8329. The loopback address ::1 expands to seven zero groups followed by 0001, and :: alone, the unspecified address, to eight zero groups.
Reverse DNS and the nibble format
PTR records for IPv6 live under ip6.arpa, and the owner name is built from the expanded address: remove the colons, reverse the 32 hex digits, put a dot between each and append ip6.arpa. For 2001:db8::1 that gives 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. Every zero has to be present, which is why you cannot work from the short form.
Delegations follow the same pattern at nibble boundaries. The reverse zone for 2001:db8:abcd::/48 is d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa, the first twelve digits of the expanded prefix in reverse. This tool gives you the expanded address to start from. It does not write the reversed name. Once the record is published, IP lookup shows the PTR hostname it finds for the address.
Sorting, storing and comparing as text
Text sorting compares character by character, so compressed addresses sort badly: 2001:db8::10 lands before 2001:db8::2, and 2001:db8::a after both. In expanded form every address has the same length and the same digit positions, so an ordinary alphabetical sort in a spreadsheet or the sort command puts them in numeric order.
The same property helps storage and matching. A CHAR(39) column holds any expanded address, a match on the first 19 characters selects everything inside one /64, and two lists can be compared with a plain diff. If the database has a native inet type or a 16-byte binary column, use that instead. Expanded text is the fallback for systems that only understand strings.
Reading subnet boundaries in the full form
Prefix lengths count bits from the left and each hex digit is four bits, so the expanded form makes the boundaries visible. A /48 ends after the third group and a /64 after the fourth. A /56 ends two digits into the fourth group and a /60 three digits in. With the zeros written out you can see which digits belong to the site prefix, which to the subnet and which to the interface.
Take fd12:3456:789a:0001:0000:0000:0000:0001. The first three groups are the /48 site prefix, 0001 is the subnet number and the last four groups are the interface identifier. For the arithmetic, IPv6 range to CIDR lists the first and last address of any prefix, and local IPv6 generator gives you a private /48 to practise on.
Accepted input and the tool's limits
Any valid IPv6 text form is accepted: compressed or already expanded, upper or lower case, in square brackets, with a dotted IPv4 tail, or with a zone suffix such as %eth0, which is dropped from the output. Output is always lowercase. Up to 500 non-empty lines are processed per paste. Later lines are ignored.
A prefix such as 2001:db8:abcd::/48 is not an address and is marked Invalid IPv6, as are a bracketed address followed by a port and a bare IPv4 address. Strip the suffix first. Invalid lines stay in the copied output as a comment starting with # invalid: so line order survives. To return to the short form, paste the list into IPv6 compression.