The libc development forked early on in the linux development, as GNU libc wasn't useable on linux systems. This was rectified in glibc 2.0, however the linux libc was up to v5, so glibc is sometimes called libc6.
You will find that a lot of programs are compiled against libc5 since it should exist on all linux machines. If you have a glibc2.1 binary you must have glibc 2.1 installed on your system. For instance, Debian is still at glibc 2.0, so you can't use any programs compiled against glibc 2.1, but you can use them if they are compiled with libc5. Pete.
