By default, iptables will display protocols, not ports. For example, let's say a rule has been added to allow connections on HTTP port 80. iptables will list HTTP, not port 80.

~]# iptables --list
Chain INPUT (policy ACCEPT 110 packets, 9880 bytes)
 target     prot opt     source              destination
ACCEPT     all  --      anywhere             172.17.0.3  tcp dpt:http

The -n or --numeric option can be used to disable DNS resolutions, so that the ports are listed, not the protocol. Likewise, notice the source now lists 0.0.0.0/0 instead of "anywhere".

~]# iptables --list --numeric
Chain INPUT (policy ACCEPT 110 packets, 9880 bytes)
 target     prot opt     source              destination
ACCEPT     all  --       0.0.0.0./0          172.17.0.3  tcp dpt:80