Let’s look at the current IPTables rules:
iptables -nvL
To open a port for the Iperf server, add the rule:
sudo iptables -A INPUT -p tcp --dport 5001 -j ACCEPT
To open a port for a specific IP or network:
sudo iptables -A INPUT -s 192.168.5.11/32 -p tcp --dport 5001 -j ACCEPT
See also my articles:
Configuring IPTables
Testing network bandwidth with Iperf