While installing or upgrading ESxi 5.1 an error might occur saying no network adapters.The cause is incompatible network adapter See the HCL of Vmware.
Resolution
Either you can customize the ISO. image using V.front customizer tool including the drivers which are available in the V front site VMware Front Experience: How to add the missing ESXi 5.0 drivers to the ESXi 5.5 installation ISO
Or by installing the compatible network adapter.
Even while upgrading the 5.1 this issue might occur though the network adapter used for preparing 5.1 host should support 5.5 but due to drivers related to the CPU make it does not support.
