Securing a Cisco Network

Part of the Cisco Networking All-in-One For Dummies Cheat Sheet

Security is always a concern, and your Cisco network needs to be properly secured. In the following sections, you see how to secure your Cisco network by configuring NAT, by configuring an ACL, and by applying that ACL.

Securing your Cisco network by configuring NAT

The following commands are used to configure NAT overload services on a router called Router1. In this example, a list of source address is created in access list #1, which is then used as the inside source list. The FastEthernet 0/0 port is the overloaded public address port that all inside addresses get translated to.

Router1>enable
Router1#configure terminal
Router1(config)#access-list 1 permit 10.0.0.0 0.255.255.255
Router1(config)#ip nat inside source list 1 interface FastEthernet 0/0 overload
Router1(config)#interface FastEthernet0/0
Router1(config-if)#ip nat outside
Router1(config-if)#interface FastEthernet0/1
Router1(config-if)#ip nat inside

Securing your Cisco network by configuring an access control list (ACL)

ACLs are used to control traffic flow. They can be used allow or deny the flow of traffic. The two main types of ACLs are:

  • Standard ACLs, which have fewer options for classifying data and controlling traffic flow than Extended ACLs. They are only able to manage traffic based on the source IP address. These ACLs are numbered from 1–99 and from 1300–1999.

  • Extended ACLs, which offer the ability to filter or control traffic based on a variety of criteria such as source or destination IP addresses, as well as protocol type such as, ICMP, TCP, UDP, or IP. These ACLs are numbered from 100–199 and from 2000–2699.

To create a standard ACL, you can use the following example which will create an ACL that allows traffic for the 192.168.8.0/24 network.

Switch1>enable
Switch1#configure terminal
Switch1(config)#access-list 50 permit 192.168.8.0 0.0.0.255

To create an extended ACL you can use the following example which will create an ACL that allows traffic with addresses in the 192.168.8.0/24 network and tcp ports of either 80 (http) or 443 (https):

Router1>enable
Router1#configure terminal
Router1(config)#access-list 101 remark This ACL is to control the outbound router traffic.
Router1(config)#access-list 101 permit tcp 192.168.8.0 0.0.0.255 any eq 80
Router1(config)#access-list 101 permit tcp 192.168.8.0 0.0.0.255 any eq 443

Securing your Cisco network by applying an access control list

After you have created an Access Control List (ACL), such as ACL 101 created above, you can apply that ACL to an interface. In the following example, this ACL is placed to restrict outbound traffic on FastEthernet0/1.

Router1>enable
Router1#configure terminal
Router1(config)#interface FastEthernet0/1
Router1(config-if)#ip access-group 101 out
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