Red Hat Enterprise Linux (RHEL) 5.7 is a minor release within the RHEL 5 lifecycle, first made generally available on July 21, 2011
Here’s the direct and correct answer:
When prompted for an installation number, you can or skip it to install the basic operating system without registering a subscription. red hat enterprise linux 5.7 x64 iso 84
http://vault.centos.org/5.7/isos/x86_64/. Note that filename includes “CentOS-5.7-x86_64-bin-DVD-1of2.iso” etc.redhat-upgrade-tool : Not recommended for production due to glibc changes, but feasible for data-only migrations.