SME Server is based on CentOS 4, which in turn is a rebuild of Red Hat Enterprise Linux 4.
* CentOS version numbers have two parts, a major version and a minor version. The major version corresponds to the version of Red Hat Enterprise Linux from which the source packages used to build CentOS are taken. The minor version corresponds to the update set of that Red Hat Enterprise Linux version from which the source packages used to build CentOS are taken. For example, CentOS 4.4 is built from the source packages from Red Hat Enterprise Linux 4 update 4. * Since mid-2006, starting with RHEL 4.4 (formerly known as RHEL 4.0 update 4), Red Hat has adopted a versioning convention identical to that of CentOS, e.g., RHEL 4.5 or RHEL 3.9.[3]