Is NULL always zero in C?

§ 6.3.2.3 of the C99 standard says

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

§ 7.17 also says

[...] NULL which expands to an implementation-defined null pointer constant [...]

The address of the NULL pointer might be different from 0, while it will behave like it was in most cases.

(This should be the same as in older C standards, which I don't have at hand right now)

The null pointer constant is always 0. The NULL macro may be defined by the implementation as a naked 0, or a cast expression like (void *) 0, or some other zero-valued integer expression (hence the "implementation defined" language in the standard).

The null pointer value may be something other than 0. When a null pointer constant is encountered, it will be converted to the proper null pointer value.

I'm assuming you mean the null pointer. It is guaranteed to compare equal to 0.¹ But it doesn't have to be represented with all-zero bits.²

See also the comp.lang.c FAQ on null pointers.

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1. See C99, 6.3.2.3.
2. There's no explicit claim; but see the footnote for C99, 7.20.3 (thanks to @birryree in the comments).