Major differences between RHEL4 & RHEL5
1.
Virtualization added in RHEL5
2.Package installation method changed to YUM(Yellow-dog Updater Modifier) in RHEL5
3.Redhat Clustring added in RHEL5
4.RHEL5 now supports unlimited RAM and Hard disk
5.Now varients in RHEL5 are
* Red Hat Enterprise Linux Advanced Platform (former AS)
* Red Hat Enterprise Linux (former ES) (limited to 2 CPU-s)
* Red Hat Enterprise Linux Desktop with Workstation and Multi-OS option
* Red Hat Enterprise Linux Desktop with Workstation option (former WS)
* Red Hat Enterprise Linux Desktop with Multi-OS option

* Red Hat Enterprise Linux Desktop (former Desktop)

Where as in RHEL4
* Red Hat Enterprise Linux AS for mission-critical/enterprise computer systems

* Red Hat Enterprise Linux ES for supported network servers
* Red Hat Enterprise Linux WS for technical power-user desktops or high- performance computing
* Red Hat Desktop ΓÇô for multiple deployments of single-user desktops

Minor differences
1.Newer kernel(2.6.18)
2.Newer softwares packages
3.Office removed from Server edition
4.More Hardware support
5.By default netconfig command is not there, we have to install it manually
6.SELinux improved a a lot in RHEL5
7.We have to provide key at the time of installation so that we can choose to install most of the packages such as Clustring/Virtualization.
8.Some of the commands changed like in
LVM

References
https://www.redhat.com/docs/manuals/enterprise/RHEL-5-manual/release-notes/package-changes.html
http://en.wikipedia.org/wiki/Red_Hat_Enterprise_Linux

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