A common point of confusion that comes up: I’m frequently asked about “firstbits”.
Often, I hear people refer to the 8 characters printed on my coins as “firstbits”, or I hear from people who worry that someone might claim the “firstbits” of their coin before it gets funded, or that they were unable to verify the funding of their coin via its “firstbits”.
It’s important to point out that I do not print “firstbits” on coins, and that this term specifically means something other than what it’s often assumed to mean.
What’s printed on my coins is the first 8 characters (Prefix) of the Bitcoin address assigned to it. This, most likely, is not the coin’s “firstbits”. Rather, given a prefix, you can get the full Bitcoin address for any of my coins from https://casascius.com/fulllist.txt – as they have all been generated in advance.
“Firstbits”, on the other hand, means a string of enough unique characters to differentiate, in a case-insensitive manner, a given Bitcoin address used on the block chain from all of the other addresses that appeared on the block chain before that Bitcoin address was first used. The definition of firstbits does not specify a certain length. Bitcoin addresses that have never been funded and do not appear in the blockchain do not have firstbits.
Firstbits are rarely 8 characters. They are only 8 characters when an address, at the time it first appeared in the block chain, needed exactly 8 characters to distinguish the address from all others on the block chain, specifically because someone else already took the matching firstbits with 7 characters. If no one has, then the firstbits for that address will always be shorter than eight characters.
Sending funds to Bitcoin addresses found by “firstbits” is dangerous, because it is very likely that any typo, no matter how trivial, will pull up a valid address that belongs to somebody else. Only when the firstbits query is exact will the correct address be returned. Many Bitcoin users are not aware that typing in too many characters could pull up a Bitcoin address that was added to the block chain later than theirs – in other words, the wrong address.