If you install Esx/Esxi 4.0 on Dell R720 server then after installation you will face a problem that your Esxi server is unable to find NIC’s. This is due to missing NIC’s driver in Esxi 4.0 version. This is a known issue in Dell R720 model.
You can follow the below mentioned steps to workaround this issue:
1: Download the required drivers from Here
2: Extract the iso image and Use WinSCP to copy the extracted files to a directory on your ESX host.
3: Use putty login to your host and change to the directory that contains the files which you just copied using WinSCP.
4: Put your host into maintenance mode
5: Run the command:
esxupdate –bundle=INT-intel-lad-ddk-igb-126.96.36.199.1-offline_bundle-185976.zip update
6: Once the process finishes, reboot your host and exit maintenance mode. Your NIC will be now active.