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The Red Hat Enterprise Linux 5 Installation Number is a 16-character hexadecimal text string used during the installation process. The installation number enables a user to install the full set of supported packages included with the subscription. For a limited set of Red Hat Enterprise Linux subscribers, the installation number is also used to activate subscriptions for additional Red Hat products and services. It replaces the Subscription Number from earlier versions of Red Hat Enterprise Linux.

Why use an installation number?

The Red Hat Enterprise Linux 5 installation trees and ISO images, in addition to the core operating system, contain a number of optional components. The Installation Number entered by the user during the installation process guides the Installer to include the correct components that match the user’s Red Hat Enterprise Linux Subscription. With certain types of Subscriptions it is also used to activate the Subscription in Red Hat Network. When registering the system to Red Hat Network (RHN), the Installation Number is used to automatically subscribe the system to the right child channels. Finally it can be used by the customer to verify compliance to the subscription and might be requested by Red Hat Support when reporting an incident.
What about installation of multiple systems?

When installing multiple systems using a Kickstart file, the Installation Number can be embedded in that Kickstart file. As Installation Numbers identify the Subscription features, they are not unique per system. Thus an identical Installation Number can be used for several systems that share the same Subscription options including Variant and SLA.
Is the use of the Installation Number mandatory?

While not recommended to do, the user has the option to skip entering the installation number or to override it in a Kickstart file. In that case the Red Hat Enterprise Linux 5 installer will only offer the core operating system components for installation, a behavior similar to the installers in earlier versions of Red Hat Enterprise Linux.

The installation number for the Red Enterprise Linux 5 subscription can be found in several places, including:

1. The new subscription activation email, received by your Red Hat Network Organization Administrator (the user who first established the Red Hat account) or your company’s purchasing contact named on the order. The subject line for the new subscription activation email is the New Subscription Activation key of the product.
2. Red Hat Subscription Management:
www.redhat.com/apps/support/IN.html
3. Red Hat Customer Service:
www.redhat.com/about/contact/dir/#custservice

Article Source: RedHat.com