Contents
- Overview
- Terminology
- Main Features
- Traffic Types Supported/Traffic Types Not Supported
- Configuration Commands
- Configuration Examples
- Feature Enhancements
- Frequently Asked Questions About NAT v3.0
Overview
In its simplest configuration, the Network Address Translator (NAT) operates on a router connecting two networks together; one of these networks (designated as inside) is addressed with either private or obsolete addresses that need to be converted into legal addresses before packets are forwarded onto the other network (designated as outside). The translation operates in conjunction with routing, so that NAT can simply be enabled on a customer-side Internet access router when translation is desired.
Use of a NAT device provides RFC 1631-style network address translation on the router platform. The goal of NAT is to provide functionality as if the private network had globally unique addresses and the NAT device was not present. RFC 1631 represents a subset of Cisco IOS NAT functionality.
Cisco IOS NAT supports "bi-directional translation" through the simultaneous use of "inside source" and "outside source" translations.
Terminology
- Inside
- The set of networks that are subject to translation.
- Outside
- All other addresses. Usually these are valid addresses located on the Internet.
- Inside local IP address
- The IP address that was assigned to a host on the inside network. The address was either globally unique but obsolete, allocated from RFC 1918 space, or just picked out of thin air. The address may or may not be globally routable; but if it is globally routable, it may actually belong to another organization.
- Inside global IP address
- The IP address of an inside host as it appears to the outside world. The address was allocated from globally unique address space, typically provided by the Internet Service Provider (ISP).
- Simple translation entry
- A translation entry which maps one IP address to another.
- Extended translation entry
- A translation entry which maps one IP address and port pair to another.
Main Features
- Static Address Translation – Telnet 207.33.94.1
The user can establish a one-to-one mapping between the inside local and global addresses.
- Dynamic Source Address Translation
The user can establish dynamic mapping between the inside local and global addresses. This is done by describing the local addresses to be translated and the pool of addresses from which to allocate global addresses, and associating the two.
- Port Address Transation (PAT)
The user can conserve addresses in the global address pool by allowing source ports in TCP connections or UDP conversations to be translated. Different local addresses then will map to the same global address, with port translation providing the necessary uniqueness. When translation is required, the new port number is picked out of the same range as the original following the convention of Berkeley Software Distributions (BSD):(1-->511, 512-->1023, 1024-->4999, 5000-->65535)
This prevents end stations from seeing connection requests with source ports apparently corresponding to the Telnet, HTTP, or FTP daemon, for example. As a result, Cisco IOS PAT supports about 4000 local addresses that can be mapped to the same global address.
- Destination Address Rotary Translation
A dynamic form of destination translation can be configured for some outside-to-inside traffic. Once a mapping is set up, a destination address matching one of those on an access list will be replaced with an address from a rotary pool. Allocation is done in a round-robin basis, performed only when a new connection is opened from the outside to the inside. All non-TCP traffic is passed untranslated (unless other translations are in effect).This feature was designed to provide protocol translation load distribution. It is not designed nor intended to be used as a substitute technology for Cisco's LocalDirector product. Destination address rotary translation should not be used to provide web service load balancing because, like vanilla DNS, it knows nothing about service availability. As a result, if a web server were to become offline, the destination address rotary translation feature would continue to send requests to the downed server.
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Traffic Types Supported
Refer to the following table for traffic types supported by Cisco IOS NAT:
| Traffic Types/Applications Supported | Traffic Types/Applications not Supported |
|---|---|
| Any TCP/UDP traffic that does not carry source and/or destination IP addresses in the application data stream | IP Multicast |
| HTTP | Routing table updates |
| TFTP | DNS zone transfers |
| telnet | BOOTP |
| archie | talk, ntalk |
| finger | SNMP |
| NTP | NetShow |
| NFS | |
| rlogin, rsh, rcp | |
| Although the following traffic types carry IP addresses in the application data stream, they are supported by Cisco IOS NAT: | |
| ICMP | |
| FTP (including PORT & PASV commands) | |
| NetBIOS over TCP/IP (datagram and name services only, session service support coming soon) | |
| Progressive Networks’ RealAudio | |
| White Pines’ CuSeeMe | |
| Xing Technologies’ StreamWorks | |
| DNS "A" and "PTR" queries | |
| H.323 [12.0(1)/12.0(1)T and later] | |
| NetMeeting [12.0(1)/12.0(1)T and later] | |
| VDOLive [11.3(4)/11.3(4)T and later] | |
| Vxtreme [11.3(4)/11.3(4)T and later] | |
Configuration Commands
Interface Configuration Commands
ip nat { inside | outside }
Interfaces need to be marked whether they are on the inside or the outside. Only packets arriving on a marked interface will be subject to translation.
Global Configuration Commands
- Defining a pool
ip nat pool <name> <start-ip> <end-ip> { netmask <netmask> | prefix-length <prefix-length> } [ type { rotary } ]- Defines a pool of addresses using start address, end address, and netmask. These addresses will be allocated as needed.
- Enabling translation of inside source addresses
ip nat inside source { list <acl> pool <name> [overload] | static <local-ip><global-ip> }- The first form enables dynamic translation. Packets from addresses that match those on the simple access list are translated using global addresses allocated from the named pool. The optional keyword overload enables port translation for UDP and TCP. The term overload is equivalent to Port Address Translation (PAT), as used on the Combinet C7x0 product.
The second form of the command sets up a single static translation.
- Enabling translation of inside destination addresses
ip nat inside destination { list <acl> pool <name> | static <global-ip> <local-ip> }- This command is similar to the source translation command. For dynamic destination translation to make any sense, the pool should be a rotary-type pool.
- Enabling translation of outside source addresses
ip nat outside source { list <acl> pool <name> | static <global-ip> <local-ip> }- The first form (list..pool..) enables dynamic translation. Packets from addresses that match those on the simple access list are translated using local addresses allocated from the named pool.
The second form (static) of the command sets up a single static translation.
- Configuring translation timeouts
ip nat translation timeout <seconds>
Dynamic translations time out after a period of non-use. When port translation is not configured, translation entries time out after 24 hours. This time can be adjusted with the above command or the following variations:ip nat translation udp-timeout <seconds> ip nat translation dns-timeout <seconds> ip nat translation tcp-timeout <seconds> ip nat translation finrst-timeout <seconds>
Exec Commands
- Showing active translations
show ip nat translations [ verbose ]
- Showing translation statistics
show ip nat statistics
- Clearing dynamic translations
- clear ip nat translation *
Clears all dynamic translations.clear ip nat translation <global-ip>
Clears a simple translation.clear ip nat translation <global-ip> <local-ip> <proto> <global-port> <local-port>
Clears a particular dynamic translation. - Debugging
debug ip nat [ <list> ] [ detailed ]
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Configuration Examples
The following sample configuration translates between inside hosts addressed from either the 192.168.1.0 or 192.168.2.0 nets to the globally-unique 171.69.233.208/28 network.
ip nat pool net-20 171.69.233.208 171.69.233.223 netmask <netmask> 255.255.255.240 ip nat inside source list 1 pool net-20 ! interface Ethernet0 ip address 171.69.232.182 255.255.255.240 ip nat outside ! interface Ethernet1 ip address 192.168.1.94 255.255.255.0 ip nat inside ! access-list 1 permit 192.168.1.0 0.0.0.255 access-list 1 permit 192.168.2.0 0.0.0.255
The next sample configuration translates between inside hosts addressed from the 9.114.11.0 net to the globally unique 171.69.233.208/28 network. Packets from outside hosts addressed from 9.114.11.0 net (the "true" 9.114.11.0 net) are translated to appear to be from net 10.0.1.0/24.
ip nat pool net-20 171.69.233.208 171.69.233.223 netmask <netmask> 255.255.255.240 ip nat pool net-10 10.0.1.0 10.0.1.255 netmask <netmask> 255.255.255.0 ip nat inside source list 1 pool net-20 ip nat outside source list 1 pool net-10 ! interface Ethernet0 ip address 171.69.232.182 255.255.255.240 ip nat outside ! interface Ethernet1 ip address 9.114.11.39 255.255.255.0 ip nat inside ! access-list 1 permit 9.114.11.0 0.0.0.255
Feature Enhancements
- The pool configuration syntax has been extended to allow discontiguous ranges of addresses. The following syntax is now allowed:
ip nat pool <name> { netmask <mask> | prefix-length <length> } [ type { rotary } ]
This command will put the user into IP NAT Pool configuration mode, where a sequence of address ranges can be configured. There is only one command in this mode:
address <start> <end>
Example:
Router(config)#ip nat pool fred prefix-length 24 Router(config-ipnat-pool)#address 171.69.233.225 171.69.233.226 Router(config-ipnat-pool)#address 171.69.233.228 171.69.233.238This configuration creates a pool containing addresses 171.69.233.225-226 and 171.69.233.228-238 (171.69.233.227 has been omitted).
- As a convenience for users wishing to translate all inside addresses to the address assigned to an interface on the router, the NAT code allows one to simply name the interface when configuring the dynamic translation rule:
ip nat inside source list <number> interface <interface> overload
If there is no address on the interface, or it the interface is not up, no translation will occur.
Example:
ip nat inside source list 1 interface Serial0 overload
- When translating addresses to an interface's address, outside-initiated connections to services on the inside network (like mail) will require additional configuration to send the connection to the correct inside host. This command allows the user to map certain services to certain inside hosts.
ip nat inside source static { tcp | udp } <localaddr> <localport> <globaladdr> <globalport>
Example:
ip nat inside source static tcp 192.168.10.1 25 171.69.232.209 25
In this example, outside-initiated connections to the SMTP port (25) will be sent to the inside host 192.168.10.1.
- The dynamic translation command can now specify a route-map to be processed instead of an access-list. A route-map allows the user to match any combination of access-list, next-hop IP address, and output interface to determine which pool to use:
ip nat inside source route-map <name> pool <name>
Example:
ip nat pool provider1-space 171.69.232.1 171.69.232.254 prefix-length 24 ip nat pool provider2-space 131.108.43.1 131.108.43.254 prefix-length 24 ip nat inside source route-map provider1-map pool provider1-space ip nat inside source route-map provider2-map pool provider2-space ! interface Serial0/0 ip nat outside ! interface Serial0/1 ip nat outside ! interface Fddi1/0 ip nat inside ! route-map provider1-map permit 10 match ip address 1 match interface Serial0/0 ! route-map provider2-map permit 10 match ip address 1 match interface Serial0/1
- The extendable keyword allows the user to configure several ambiguous static translations, where an ambiguous translations are translations with the same local or global address.
ip nat inside source static <localaddr> <globaladdr> extendable
Some customers want to use more than one service provider and translate into each provider's address space. You can use route-maps to base the selection of global address pool on output interface as well as an access-list match. Following is an example:
ip nat pool provider1-space ... ip nat pool provider2-space ... ip nat inside source route-map provider1-map pool provider1-space ip nat inside source route-map provider2-map pool provider2-space ! route-map provider1-map permit 10 match ip address 1 match interface Serial0/0 ! route-map provider2-map permit 10 match ip address 1 match interface Serial0/1 . . .
Once that is working, they might also want to define static mappings for a particular host using each provider's address space. The software does not allow two static translations with the same local address, though, because it is ambiguous from the inside. The router will accept these static translations and resolve the ambiguity by creating full translations (all addresses and ports) if the static translations are marked as "extendable". For a new outside-to-inside flow, the appropriate static entry will act as a template for a full translation. For a new inside-to-outside flow, the dynamic route-map rules will be used to create a full translation.
- Many customers want to configure the NAT software to translate their local addresses to global addresses allocated from unused addresses from an attached subnet. This requires that the router answer ARP requests for those addresses so that packets destined for the global addresses are accepted by the router and translated. (Routing takes care of this packet delivery when the global addresses are allocated from a virtual network which isn't connected to anything.) When a NAT pool used as an inside global or outside local pool consists of addresses on an attached subnet, the software will generate an alias for that address so that the router will answer ARPs for those addresses.
This automatic aliasing also occurs for inside global or outside local addresses in static entries. It can be disabled for static entries can be disabled by using the "no-alias" keyword:.
ip nat inside source static <local-ip-address> <global-ip-address> no-alias
- For ease of network management, some sites wish to translate prefixes, not addresses. That is, they wish the translated address to have the same host number as the untranslated address. Of course, the two prefixes must be of the same length. This feature can be enabled by configuring dynamic translation as usual, but configuring the address pool to be of type "match-host":
ip nat pool fred <start> <end> prefix-length <len> type match-host
- The following new timeouts have been implemented for extended translation entries:
ip nat translation ?
- icmp-timeout Specify timeout for NAT ICMP flows
syn-timeout Specify timeout for NAT TCP flows after a SYN and no further data
- Using the following command, Cisco IOS NAT can be configured to limit the number of translation entries it creates. The default is that there is no limit.
ip nat translation max-entries <n>
Frequently Asked Questions About NAT v3.0
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For More Information
NAT
Visit the main CCO NAT Page.
Refer to the "Configure Network Address Translation (NAT)" section in the Configuring IP Addressing documentation.
RFCs
For information about the RFCs referenced in this document, see:
- Primary and Secondary RFC Repositories, a Technical Tip that explains where and how to obtain RFCs.
- List of Cisco-Supported RFCs, a list of RFCs supported by Cisco's system software as of release 10.0(1).
- RFCs, Standards and Technical Publications, an index of requests for comment.
White Papers
For information about several applications for Cisco IOS, including sample configurations, see:
- Expanding Enterprise Connectivity with Net Address Translation [1.6M] PowerPoint presentation - 06/16/1998
- This PowerPoint presentation gives an overview of NAT by describing its benefits, its availability and platform support, terminology, and types of translations. Configurations and examples are included.
- Enabling Enterprise Multihoming with Cisco IOS NAT - Praveen Akkiraju, Yakov Rekhter, Kevin Delgadillo
- Explains how to configure and use the Border Gateway Protocol (BGP), Network Address Translation (NAT), and the Domain Name System (DNS) together to enable redundant multihomed enterprise connectivity, eliminate host renumbering requirements when changing ISPs, and reduce costs associated with IP address management tasks.