Why does the client have a listen port?

    nsh config init -c server.yaml -n server --listen-port=51820 --ip=172.21.248.1
    nsh config init -c client.yaml -n client --listen-port=51821 --ip=172.21.248.2

Typically, servers have easiest to configure NAT/firewall/port forwarding and clients (to me) are basically thought of as "not going to get through their router/firewall configuration no matter what".