Etar, if you are talking about 100 bitcoin challenge competition, all the addresses have the pattern: their 1st (big endian) bit always equals to '1'. You can see for every found private key and present it in binary format, and you can see that the 1st bit always '1' (i mean the 1st of unknown part, without leading zeros).
For example, the key for 80bit address was 0xEA1A5C66DCC11B5AD180, and it is in binary:
11101010000110100101110001100110110111001100000100011011010110101101000110000000
The 1st is '1'.
HardwareCollector absolutely right saying that 80bit address has 79bit search range. And this applies to any other key: n-bit address has (n-1) bit search range.
But every key in 80 bit range will have a "1" at the 80 bit marker, or else it would be a different bit. Turn that 1 into a 0 and if it's followed by a 1, then it's a 79 bit key.
The puzzle creator had to make the last bit always 1, otherwise it would be possible that two private keys would be in the same search range when doing a sequential brute-force search. Making the high bit 1 guarantees that the next puzzle will be 1 bit longer than the previous.