We are planning to create an NFS datastore on 6.5 ESXi hosts temporarily and it will be connected from mgmt IP to NAS storage. Both IPs are configured with different subnets and there is a firewall in between, please let me know what are ports need to open in order to access NAS. Also, let me know if it is fine to access the NAS on another subnet.
NFS is not a "VMware thing" so I would think the better place to find this answer would be the Internet for default, but more importantly, your specific configuration files. Perhaps the config was modified to use non-standard ports. It also depends on which version of NFS you are using. With that said, you should be able to mount an NFS datastore from behind a firewall given the policy is properly implemented. I would also consider checking the firewall to make sure that it can handle the traffic flow and won't cause a bottle neck for your VMs.