Switch(config)# access-list 102 permit icmp any any
Switch(config)# interface gigabitethernet2/0/1
Switch(config-if)# ip access-group 102 in
In this example, suppose that you have a network connected to the Internet, and you want any host on the
network to be able to form TCP connections to any host on the Internet. However, you do not want IP hosts
to be able to form TCP connections to hosts on your network, except to the mail (SMTP) port of a dedicated
mail host.
SMTP uses TCP port 25 on one end of the connection and a random port number on the other end. The same
port numbers are used throughout the life of the connection. Mail packets coming in from the Internet have
a destination port of 25. Outbound packets have the port numbers reversed. Because the secure system of the
network always accepts mail connections on port 25, the incoming and outgoing services are separately
controlled. The ACL must be configured as an input ACL on the outbound interface and an output ACL on
the inbound interface.
Switch(config)# access-list 102 permit tcp any 128.88.0.0 0.0.255.255 eq 23
Switch(config)# access-list 102 permit tcp any 128.88.0.0 0.0.255.255 eq 25
Switch(config)# interface gigabitethernet1/0/1
Switch(config-if)# ip access-group 102 in
In this example, the network is a Class B network with the address 128.88.0.0, and the mail host address is
128.88.1.2. The established keyword is used only for the TCP to show an established connection. A match
occurs if the TCP datagram has the ACK or RST bits set, which show that the packet belongs to an existing
connection. Gigabit Ethernet interface 1 on stack member 1 is the interface that connects the router to the
Internet.
Switch(config)# access-list 102 permit tcp any 128.88.0.0 0.0.255.255 established
Switch(config)# access-list 102 permit tcp any host 128.88.1.2 eq 25
Switch(config)# interface gigabitethernet1/0/1
Switch(config-if)# ip access-group 102 in
Examples: Named ACLs
Creating named standard and extended ACLs
This example creates a standard ACL named internet_filter and an extended ACL named marketing_group.
The internet_filter ACL allows all traffic from the source address 1.2.3.4.
Switch(config)# ip access-list standard Internet_filter
Switch(config-ext-nacl)# permit 1.2.3.4
Switch(config-ext-nacl)# exit
The marketing_group ACL allows any TCP Telnet traffic to the destination address and wildcard 171.69.0.0
0.0.255.255 and denies any other TCP traffic. It permits ICMP traffic, denies UDP traffic from any source to
the destination address range 171.69.0.0 through 179.69.255.255 with a destination port less than 1024, denies
any other IP traffic, and provides a log of the result.
Switch(config)# ip access-list extended marketing_group
Switch(config-ext-nacl)# permit tcp any 171.69.0.0 0.0.255.255 eq telnet
Switch(config-ext-nacl)# deny tcp any any
Switch(config-ext-nacl)# permit icmp any any
Switch(config-ext-nacl)# deny udp any 171.69.0.0 0.0.255.255 lt 1024
Switch(config-ext-nacl)# deny ip any any log
Switch(config-ext-nacl)# exit
Catalyst 2960-X Switch Security Configuration Guide, Cisco IOS Release 15.0(2)EX
184 OL-29048-01
Configuring IPv4 ACLs
IPv4 ACL Configuration Examples
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