User's Manual

Table Of Contents
423
Users Manual of CS-6306R
43.4.2.4 Extensible Access List Example
In the following example, the first line allows any new TCP to connect the destination port after port 1023. The
second line allows any new TCP to connect the SMTP port of host 130.2.1.2.
ip access-list extended aaa
permit tcp any 130.2.0.0 255.255.0.0 gt 1023
permit tcp any 130.2.1.2 255.255.255.255 eq 25
interface vlan 10
ip access-group aaa in
Another example to apply the extensible access list is given. Suppose a network connects the Internet, you
expect any host in the Ethernet can create TCP connection with the host in the Internet. However, you expect
the host in the Internet cannot create TCP connection with the host in the Ethernet unless it connects the
SMTP port of the mail host.
During the connection period, the same two port numbers are used. The mail packet from the Internet has a
destination port, that is, port 25. The outgoing packet has a contrary port number. In fact, the security system
behind the routing switch always receives mails from port 25. That is the exact reason why the incoming
service and the outgoing service can be uniquely controlled. The access list can be configured as the
outgoing service or the incoming service.
In the following case, the Ethernet is a B-type network with the address 130.20.0.0. The address of the mail
host is 130.20.1.2. The keyword established is only used for the TCP protocol, meaning a connection is
created. If TCP data has the ACK or RST digit to be set, the match occurs, meaning that the packet belongs to
an existing connection.
ip access-list aaa
permit tcp any 130.20.0.0 255.255.0.0 established
permit tcp any 130.20.1.2 255.255.255.255 eq 25
interface vlan 10
ip access-group aaa in