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Here's a table I got from bitcoin wiki.
They say:
A special case, leading numbers 1 (one) is especially difficult.
But why?
Why is it harder to generate addresses starting with 1s?
Answers
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An address has 1 + 20 + 4 bytes, which has a total of 200 bits. Each of the Base58 characters contain log_2 58 bits of information, which is approximately 5.858. If one created an address that has the public key hash 0000...00, the result would be a 5-6 character address, due to the checksum. To prevent this, Satoshi decided to add "1"s for each 0x00 byte in front of the address. Those "1"s each contain 8 bits of information instead of 5.85... As they carry more information, it's harder to find an address starting with those 1s.
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Take a look at this answer. http://bitcoin.stackexchange.com/questions/10828/vanitygen-why-is-it-easier-to-generate-an-address-that-starts-with-a-capital-le
– m1xolyd1an – 2016-08-28T15:35:18.517See this question's answer here
– user2716556 – 2016-10-25T19:25:10.807