Filtering IP Packets on Router Interfaces with Extended Access Lists

Suppose a router is connected to an "internal" Ethernet network and also has a link to the Internet via its serial 0 interface. The internal Ethernet network is the Class B network 131.108.0.0. You want to allow Internet Control Message Protocol (ICMP) messages in from the Internet to the Ethernet network for error-reporting purposes. You also want to allow TCP packets in from the Internet if they are destined to the Simple Mail Transport Protocol (SMTP) port of host 131.108.15.1 or if they are destined to ports greater that 1023 (this setup will allow TCP packets that are in response to connections generated from the internal network). This setup can be accomplished with the following extended access list:

access-list 177 permit tcp 0.0.0.0 255.255.255.255 131.108.0.0 0.0.255.255    gt 1023
access-list 177 permit tcp 0.0.0.0 255.255.255.255 131.108.15.1 0.0.0.0 eq    25
access-list 177 permit icmp 0.0.0.0 255.255.255.255 131.108.0.0 0.0.255.255

interface s 0
ip address 207.200.115.6 255.255.255.252
ip access-group 177 in

This access list could also be written as:

access-list 177 permit tcp any 131.108.0.0 0.0.255.255 gt 1023
access-list 177 permit tcp any host 131.108.15.1 eq smtp
access-list 177 permit icmp any 131.108.0.0 0.0.255.255

We could also accomplish the same thing with the following standard named access list:

ip access-list extended filter-in
permit tcp any 131.108.0.0 0.0.255.255 gt 1023
permit tcp any host 131.108.15.1 eq smtp
permit icmp any 131.108.0.0 0.0.255.255

interface s 0
ip address 207.200.115.6 255.255.255.252
ip access-group filter-in in