Why does Red Hat Enterprise Linux 5 32bit PAE detects only 64Gib of memory?

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Issue

RHEL 5.6 32bit installation on a new Dell R710 with 128GB of RAM is only showing 64GB of RAM.
Why RHEL 5 PAE kernel detects upto 64 Gib of memory ?

Environment

Red Hat Enterprise Linux 5
Red Hat Enterprise Linux 6
32bit x86
PAE Kernel

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