In ACL what is the use of allowing IP address by specific ports (Example: eq 443/80/21 and so on)
In Access Control Lists (ACLs), you can allow or deny traffic based on the source and destination IP addresses or port numbers. An extended access control list will allow you to deny or permit traffic from specific IP addresses and ports. It also gives you the ability to control the type of protocol that can be transferred such as ICMP, TCP, UDP and so forth.
For example, if you want to allow traffic from a specific IP address on port 80, you can use the following command:
access-list 101 permit tcp host 192.168.1.1 eq 80 any
This command will allow traffic from the IP address 192.168.1.1 on port 80, i.e. HTTP services.
Similarly, port no. 443/80/21 will help you to control specific traffic HTTPS/HTTP/FTP for specific IP addresses.
Please refer the attached pic for some well known ports that can be used to control specific traffic for specific IP addresses.