Why glibc binary is called libc.so.6 not a libc.so.1 or libc.so.4?

Question

The modern glibc binary is called libc.so.6 in Linux. Why is "6" used here? The libc.so.1 or libc.so.8 can be good names too IMHO.

Wikipedia gives some history at http://en.wikipedia.org/wiki/GNU_C_Library but doesn't explain fully

In the early 1990s, the developers of the Linux kernel forked glibc. Their fork, called "Linux libc", was maintained separately for years and released versions 2 through 5.

When FSF released glibc 2.0 in January 1997, .... At this point, the Linux kernel developers discontinued their fork and returned to using FSF's glibc.[6]

The last used version of Linux libc used the internal name (soname) libc.so.5. Following on from this, glibc 2.x on Linux uses the soname libc.so.6

Answer 1

Those SONAMEs have already been used. Since the new version of the library presents a break in the interface, a new major SONAME number must be used for it.