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Cisco IOS: It's Not Just for Routing Anymore
May 31, 1999
Additional Information
Related Links
Network Computing, Feb. 22, 1999
"The Cost of Security on Cisco Routers"

Network Computing, Feb. 8, 1999
"Cisco 1720 Ensures Secure Links"

Network Computing, April 15, 1998
"Demystifying Cisco Access Control Lists"

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By Greg Shipley  Cisco Systems' Internetwork Operating System (IOS), the software that runs on the vendor's routers, has become the network administrator's Swiss Army knife. We found that IOS version 12, the most recent revision, continues to improve upon fundamental routing principles and is chock-full of new features--NAT (Network Address Translation) support and expanded ACL (access control list) and VPN functionality, for starters--that go beyond basic routing. Knowing how to leverage these features can save an administrator much time and effort.

On the security front, IOS has supported ACLs for IP, SAP, IPX and other protocols. Version 12 adds NAT, which had been available only in traditional firewall products; it lets administrators map internal/hidden IP addresses to external/public ones. This functionality is useful to organizations using RFC1918-based addressing, as well as those needing quick fixes for address-duplication conflicts between remote sites. With NAT, administrators have the choice to hide an entire network behind a single address/interface. Known as IP masquerading--or PAT (Port Address Translation) in Cisco-speak--this technique is helpful to remote offices that use an ISDN connection for inexpensive Internet access.

We configured a Cisco 3640 router running IOS 12 for NAT (also available in some builds of the IOS version 11.x firewall edition) with little difficulty. First, we simply defined our NAT table within the global configuration mode:

ip nat inside source list 1 interface Serial1 overload

!

access-list 1 permit 10.9.100.0 0.0.0.255

We then defined the NAT table within the interfaces of the internal and external networks:

interface Ethernet0/0

ip address 10.9.100.254 255.255.255.0

ip nat inside

interface Serial1/0

ip nat outside

The above configuration lets any host on the Ethernet segment 10.9.100.0/24 masquerade as the serial interface's IP address. In this case, we configured the serial port for dial-on-demand routing, and we were able to "hide" an entire RFC1918-based, Class C address range behind a dynamically assigned PPP address (see "Using NAT," to the right).


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