Why does the "showmount" command not work on either an NFS client or from the same NFS server?

Issue

• A Network File System (NFS) server daemon is running without errors
• Running showmount -e my-server does not work on either a NFS client or from the same NFS server
• The error received from a NFS client is, "clnt_create: RPC: Program not registered"
• The error received from the NFS server is, "clnt_create: RPC: Remote system error - Address family not supported by protocol"