The only thing that is not defined is comparing a pointer one-past-the-end to a pointer to the very beginning of a toplevel object. Apart from this rule, pointers of course do not need to be derived from the same object in order to be compared with == and !=.

&a + 1 == &b is unspecified: it may produce 0 or 1, and it may not produce the same result if you evaluate it several times.

Similarly, if both the char pointers p and q were obtained with malloc(10), after they have been tested for NULL, all these operations are valid:

  p == q (false)
  p + 1 == q (false)
  p + 1 == q + 1 (false)
  p + 10 == q + 1 (false)

Only p+10 == q and p == q+10 are unspecified (of the comparisons that can be built without invoking UB during the pointer arithmetic itself).

I have no idea what led that person to (apparently) write that &a==&b is undefined. This is plain wrong. I do not see any ambiguity in the relevant clause (https://port70.net/~nsz/c/c11/n1570.html#6.5.9p6 ). Yes, the standard is in English and natural languages are ambiguous, but you might as well claim that a+b is undefined because the standard does not define what the word “sum” means (https://port70.net/~nsz/c/c11/n1570.html#6.5.6p5 ).