iptables block access to port 8000 except from IP address:
iptables -A INPUT -p tcp --dport 8000 -s 1.2.3.4 -j ACCEPT
iptables -A INPUT -p tcp --dport 8000 -j DROP
Alternatively:
sudo iptables -A INPUT -p tcp --dport 8000 ! -s 1.2.3.4 -j DROP
[note the relative position of the exclamation (negation) sign]