macOS Sierra: mounting NFS volumes
The accepted answer only works when the NFS server allows connections on non-reserved ports (i.e. >1024).

The exports man pages say it:


  exportfs understands the following export options: secure
  
  This option requires that requests originate on an Internet port less
  than IPPORT_RESERVED (1024). This option is on by default. To turn it
  off, specify insecure.


For my CentOS 7.3 box this is not the case by default and I had to add insecure to the mount. 

So for MACOS client to work you'll have to add the insecure option to your nfs server in your export

E.g.: /my/export 192.168.5.0/24(insecure)

If that's not possible: using mountyou can supply an option to allow reserved ports from the Mac side:
sudo mount -t nfs -o resvport server:/my/export nfs