& by itself like that is a bitwise and, and 4095 is one away from being 4096, a nice round binary number. In binary, 4095 looks like:

Code:

0000111111111111

All the higher-order bits, where 4095 is zero, get turned off, only the lower-order bits get let through.

C supports octal directly, though. Numbers beginning with a 0 are assumed to be octal. The value of 0777 is the same as that of 777 in the chmod sense, and you can get the same value as 4095 with the less-inexplicable octal value of 07777.