How does the cisco router set access only to specific ports?

With extended ACL, you can:

Router (Configuration) #ip Access List Extended Text

//Defines the ACL naming text.

Router (config-ACL) # PermitTCP (UDP) Source IP Wildcard Mask [eq Port Number] Destination IP Wildcard Mask eq Port Number.

//Define the entries of ACL, allowing specific access port numbers.

Router (configuration) #int f0/0 (interface number)

Router (Configuration -if)#ip Access Group Text Input (Output)

//The interface calls an ACL named TEXT.

1. For example, any IP is allowed to access TCP port 80(HTTP port) on interface F0/0, and the rest are filtered.

Ip access list extension per_HTTP

Allow tcp any any eq http

Reject ip any any

export

int f0/0

Ip access group per_HTTP in

2. For example, interface F0/0 blocks1.1to access tcp and udp 139 ports in 2.2.2.2, and the rest of the traffic is released:

The access list 199 deny TCP host 1. 1. 1 host 2.2.2.2 eq139//ACL another method.

The access list 199 denies udp host1.1.1.1host 2.2.2.2 eq139.

Access-list 199 allows any any//ACL to implicitly deny any any by default.

int f0/0

Ip access-group 199 in