Chances are the VMNet you are using has VMware's DCHP server enabled by default.
To disable DHCP for a specific VMNet: (VMNet 1 in this example)

VMWare Server hosted on Linux
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Edit the /etc/vmware/locations file and change the following lines
answer VNET_1_DHCP yes
to
answer VNET_1_DHCP no

VMWare Server hosted on Windows
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Stop the VMWare DHCP Service

VMWare Infrastructure (ESX, ESX3i)
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You must configure a second vSwitch solely for the private use of the cluster. Name it "Private Cluster Network". Associate the second vNIC on the master and the primary vNICs on the nodes with it.