Layer 3 Network Address Translation

Network Address Translation

Network Address Translation (NAT) is designed for IP address conservation. It enables private IP networks that use unregistered IP addresses to connect to the Internet. NAT operates on a device, usually connecting two networks together, and translates the private (not globally unique) addresses in the internal network into global routable addresses. It does so before packets are forwarded onto another network.

NAT can be configured to advertise only one address for the entire network to the outside world. This ability provides more security by effectively hiding the entire internal network behind that one address. NAT offers the dual functions of security and address conservation and is typically implemented in remote-access environments.

NAT is also used at the enterprise edge to allow internal users access to the Internet and to allow Internet access to internal devices such as mail servers.


Note

Layer 3 Network Address Translation requires the Network Advantage license. For the latest license requirements, see the Cisco IE3500 Rugged Series Data Sheet.

Finding Feature Information

Your software release may not support all the features described in this document. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this chapter.

Use the Cisco Feature Navigator to find information about platform support and Cisco software image support. To access the Cisco Feature Navigator, go to https://cfnng.cisco.com/. An account on Cisco.com is not required.

Benefits of Configuring NAT

Configuring NAT provides the following benefits:

  • NAT Resolves the problem of IP depletion.

    NAT allows organizations to resolve the problem of IP address depletion when they have existing networks and need to access the Internet. Sites that do not yet possess Network Information Center (NIC)-registered IP addresses must acquire IP addresses. In such cases, if more than 254 clients are present or are planned, the scarcity of Class B addresses becomes a serious issue. NAT addresses these issues by mapping thousands of hidden internal addresses to a range of easy-to-get Class C addresses.

  • NAT provides a layer of security by preventing the client IP address from being exposed to the outside network.

    Sites that already have registered IP addresses for clients on an internal network may want to hide those addresses from the Internet so that hackers cannot directly attack clients. With client addresses hidden, a degree of security is established. NAT gives LAN administrators complete freedom to expand Class A addressing, which is drawn from the reserve pool of the Internet Assigned Numbers Authority. The expansion of Class A addresses occurs within the organization without a concern for addressing changes at the LAN or the Internet interface.

  • Cisco software can selectively or dynamically perform NAT. This flexibility allows network administrator to use RFC 1918 addresses or registered addresses.

  • NAT is designed for use on a variety of devices for IP address simplification and conservation. In addition, NAT allows the selection of internal hosts that are available for translation.

  • A significant advantage of NAT is that it can be configured without requiring any changes to devices other than to those few devices on which NAT will be configured.

How NAT Works

A device that is configured with NAT has at least one interface to the inside network and one to the outside network. In a typical environment, NAT is configured at the exit device between a stub domain and the backbone. When a packet leaves the domain, NAT translates the locally significant source address into a globally unique address. When a packet enters the domain, NAT translates the globally unique destination address into a local address.

Multiple inside networks could be connected to the device and similarly there might exist multiple exit points from the device towards outside networks. If NAT cannot allocate an address because it has run out of addresses, it drops the packet and sends an Internet Control Message Protocol (ICMP) host unreachable packet to the destination.

Translation and forwarding are performed in the hardware switching plane, improving the overall throughput performance. For more details on performance, see the section Performance and Scale Numbers for NAT.

Figure 1. NAT

Uses of NAT

You can use NAT in the following scenarios:

  • To connect to the Internet when only a few of your hosts have globally unique IP address.

    NAT is configured on a device at the border of a stub domain (referred to as the inside network) and a public network such as the Internet (referred to as the outside network). NAT translates internal local addresses to globally unique IP addresses before sending packets to the outside network.

    As a solution to the connectivity problem, NAT is practical only when relatively few hosts in a stub domain communicate outside of the domain at the same time. In such cases, only a small subset of the IP addresses in the domain must be translated into globally unique IP addresses when outside communication is necessary, and these addresses can be reused.

  • To renumber:

    Instead of changing the internal addresses, which can be a considerable amount of work, you can translate them by using NAT.

NAT Inside and Outside Addresses

The term inside in a NAT context refers to networks owned by an organization that must be translated. When NAT is configured, hosts within this network have addresses in one space (known as the local address space) that appears to those outside the network as being in another space (known as the global address space).

Similarly, the term outside refers to those networks to which the stub network connects, and which are generally not under the control of an organization. Hosts in outside networks can also be subject to translation, and can thus have local and global addresses.

NAT uses the following definitions:

  • Inside local address: an IP address that is assigned to a host on the inside network. The address is probably not a routable IP address assigned by NIC or service provider.

  • Inside global address: a global routable IP address (assigned by the NIC or service provider) that represents one or more inside local IP addresses to the outside world.

  • Outside local address: the IP address of an outside host as it appears to the inside network. Not necessarily a routable IP address, it is allocated from the address space that is routable on the inside.

  • Outside global address: the IP address assigned to a host on the outside network by the owner of the host. The address is allocated from a globally routable address or network space.

  • Inside Source Address Translation: translates an inside local address to inside global address.

  • Outside Source Address Translation: translates the outside global address to outside local address.

  • Static Port Translation: translates the IP address and port number of an inside/outside local address to the IP address and port number of the corresponding inside/outside global address.

  • Static Translation of a given subnet: translates a specified range of subnets of an inside/outside local address to the corresponding inside/outside global address.

  • Half Entry: represents a mapping between the local and global address/ports and is maintained in the translation database of NAT module. A half entry may be created statically or dynamically based on the configured NAT rule.

  • Full Entry/Flow entry: represents a unique flow corresponding to a given session. In addition to the local to global mapping, it also maintains the destination information which fully qualifies the given flow. A Full entry is always created dynamically and maintained in the translation database of NAT module.

VRF-Aware Network Address Translation

NAT is typically configured to operate across the default or global routing domain. As per this feature, the inside and outside NAT domains are associated with the default VRF space and the translations are effected accordingly. However, there are certain scenarios where NAT is required to operate in a VRF setting. One common scenario involves enabling shared service access for private networks that have overlapping address space. In such cases, the given private networks can be placed in different VRFs and global service access can be achieved by configuring VRF-aware NAT rules that map overlapping private address to unique global address. VRF-awareness enables NAT to carry out address and port translation by taking the VRF of the private networks into consideration.

VRF-aware NAT supports only VRF to Global translation of IP addresses. VRF to Global translation is between a NAT inside interface that is associated with a specific VRF and a NAT outside interface that is associated with the global VRF. Intra-VRF NAT translation (which involves the NAT-inside and NAT-outside interfaces of the same specific VRF) and Inter-VRF NAT translation (which involves NAT-inside and NAT-outside interfaces that are associated with different VRFs) are not supported. NAT behavior is undefined in such unsupported scenarios. We recommend that you deploy only the VRF to Global NAT translation in your network.

Types of NAT

You can configure NAT such that it advertises only a single address for your entire network to the outside world. The configuration effectively hides the internal network from the world, giving you some additional security.

The types of NAT include:

  • Static address translation (static NAT): Allows one-to-one mapping between local and global addresses.

  • Dynamic address translation (dynamic NAT): Maps unregistered IP addresses to registered IP addresses from a pool of registered IP addresses.

  • Overloading / PAT: Maps multiple unregistered IP addresses to a single registered IP address (many to one) using different Layer 4 ports. This method is also known as Port Address Translation (PAT). By using overloading, thousands of users can be connected to the Internet by using only one real global IP address.

Using NAT to Route Packets to the Outside Network (Inside Source Address Translation)

You can translate unregistered IP addresses into globally unique IP addresses when communicating outside your network.

You can configure static or dynamic inside source address translation as follows:

  • Static translation establishes a one-to-one mapping between the inside local address and an inside global address. Static translation is useful when a host on the inside must be accessible by a fixed address from the outside. You can enable Static translation by configuring a static NAT rule as explained in the x section.

  • Dynamic translation establishes a mapping between an inside local address and a pool of global addresses dynamically. Dynamic translation can be enabled by configuring a dynamic NAT rule and the mapping is established based on the result of the evaluation of the configured rule at run-time. You can employ an Access Control List (ACL), both Standard and Extended ACLs, to specify the inside local address. The inside global address can be specified through an address pool or an interface. Dynamic translation is enabled by configuring a dynamic rule as explained in the section Configuring Dynamic Translation of Inside Source Addresses.

The following figure illustrates a device that is translating a source address inside a network to a source address outside the network.

Figure 2. NAT Inside Source Translation

The following process describes the inside source address translation, as shown in the preceding figure:

  1. The user at host 10.1.1.1 opens a connection to Host B in the outside network.

  2. NAT module intercepts the corresponding packet and attempts to translate the packet.

    The following scenarios are possible based on the presence or absence of a matching NAT rule:

    • If a matching static translation rule exists, the packet gets translated to the corresponding inside global address. Otherwise, the packet is matched against the dynamic translation rule, and in the event of a successful match, it gets translated to the corresponding inside global address. The NAT module inserts a fully qualified flow entry corresponding to the translated packet, into its translation database. This facilitates fast translation and forwarding of the packets corresponding to this flow, in either direction.

    • The packet gets forwarded without any address translation in the absence of a successful rule match.

    • The packet is dropped in the event of failure to obtain a valid inside global address even-though we have a successful rule match.


      Note

      If an ACL is employed for dynamic translation, NAT evaluates the ACL and ensures that only the packets that are permitted by the given ACL are considered for translation.

  3. The device replaces the inside local source address of host 10.1.1.1 with the inside global address of the translation, 203.0.113.2, and forwards the packet.

  4. Host B receives the packet and responds to host 10.1.1.1 by using the inside global IP destination address (DA) 203.0.113.2.

  5. The response packet from host B would be destined to the inside global address. The NAT module intercepts this packet and translates it back to the corresponding inside local address with the help of the flow entry that has been set up in the translation database.

Host 10.1.1.1 receives the packet and continues the conversation. The device performs Step 2 to Step 5 for each packet that it receives.

Outside Source Address Translation

You can translate the source address of the IP packets that travel from outside of the network to inside the network. This type of translation is usually employed in conjunction with inside source address translation to interconnect overlapping networks.

This process is explained in the section Configuring Translation of Overlapping Networks.

Port Address Translation

You can conserve addresses in the inside global address pool by allowing a device to use one global address for many local addresses. This type of NAT configuration is called overloading or port address translation (PAT).

When overloading is configured, the device maintains enough information from higher-level protocols (for example, TCP or UDP port numbers) to translate the global address back to the correct local address. When multiple local addresses map to one global address, the TCP or UDP port numbers of each inside host distinguish between the local addresses.

The following figure illustrates a NAT operation when an inside global address represents multiple inside local addresses. The TCP port numbers act as differentiators.

Figure 3. PAT / NAT Overloading Inside Global Addresses

The device performs the following process in the overloading of inside global addresses, as shown in the figure above. Both Host B and Host C believe that they are communicating with a single host at address 203.0.113.2. However, they are actually communicating with different hosts; the port number is the differentiator. In fact, many inside hosts can share the inside global IP address by using many port numbers.

  1. The user at host 10.1.1.1:1723 opens a connection to Host B and the user at host 10.1.1.2:1723 opens a connection to Host C.

  2. NAT module intercepts the corresponding packets and attempts to translate the packets.

    Based on the presence or absence of a matching NAT rule the following scenarios are possible:

    • If a matching static translation rule exists, then it takes precedence and the packets are translated to the corresponding global address. Otherwise, the packets are matched against dynamic translation rule and in the event of a successful match, they are translated to the corresponding global address. NAT module inserts a fully qualified flow entry corresponding to the translated packets, into its translation database, to facilitate fast translation and forwarding of the packets corresponding to this flow, in either direction.

    • The packets are forwarded without any address translation in the absence of a successful rule match.

    • The packets are dropped in the event of failure to obtain a valid inside global address even though we have a successful rule match.

    • Because this is a PAT configuration, transport ports help translate multiple flows to a single global address. (In addition to source address, the source port is also subjected to translation and the associated flow entry maintains the corresponding translation mappings.)

  3. The device replaces inside local source address/port 10.1.1.1/1723 and 10.1.1.2/1723 with the corresponding selected global address/port 203.0.113.2/1024 and 203.0.113.2/1723 respectively and forwards the packets.

  4. Host B receives the packet and responds to host 10.1.1.1 by using the inside global IP address 203.0.113.2, on port 1024. Host C receives the packet and responds to host 10.1.1.2 using the inside global IP address 203.0.113.2, on port 1723.

  5. When the device receives the packets with the inside global IP address, it performs a NAT table lookup; the inside global address and port, and the outside address and port as keys; translates the addresses to the inside local addresses 10.1.1.1:1723 / 10.1.1.2:1723 and forwards the packets to host 10.1.1.1. and 10.1.1.2 respectively.

Host 10.1.1.1 and Host 10.1.1.2 receive the packet and continue the conversation. The device performs Step 2 to Step 5 for each packet it receives.

Overlapping Networks

Use NAT to translate IP addresses if the IP addresses that you use are not legal or officially assigned. Overlapping networks result when you assign an IP address to a device on your network that is already legally owned and assigned to a different device on the Internet or outside the network.

The following figure depicts overlapping networks: the inside network and outside network both have the same local IP addresses (10.1.1.x). You need network connectivity between such overlapping address spaces with one NAT device to translate the address of a remote peer (10.1.1.3) to a different address from the perspective of the inside.

Figure 4. NAT Translating Overlapping Addresses

Notice that the inside local address (10.1.1.1) and the outside global address (10.1.1.3) are in the same subnet. To translate the overlapping address, first, the inside source address translation happens with the inside local address getting translated to 203.0.113.2 and a half entry is created in the NAT table. On the Receiving side, the outside source address is translated to 172.16.0.3 and another half entry is created. The NAT table is then updated with a full entry of the complete translation.

The following steps describe how a device translates overlapping addresses:

  1. Host 10.1.1.1 opens a connection to 172.16.0.3.

  2. The NAT module sets up the translation mapping of the inside local and global addresses to each other and the outside global and local addresses to each other.

  3. The Source Address (SA) is replaced with inside global address and the Destination Address (DA) is replaced with outside global address.

  4. Host C receives the packet and continues the conversation.

  5. The device does a NAT table lookup, replaces the DA with inside local address, and replaces the SA with outside local address.

  6. Host 10.1.1.1 receives the packet and the conversation continues using this translation process.

Limitations of NAT

  • Some NAT operations are currently not supported in the hardware data plane. The following are such operations that are carried out in the relatively slower software data plane:

    • Translation of Internet Control Message Protocol (ICMP) packets

    • Translation of packets that require application layer gateway (ALG) processing

    • Packets that require both inside and outside translation

  • The maximum number of sessions that can be translated and forwarded in the hardware in an ideal setting is limited to 240. Additional flows that require translation are handled in the software data plane at a reduced throughput.


    Note

    Each translation consumes two entries in TCAM.

  • A configured NAT rule might fail to get programmed into the hardware owing to resource constraint. This could result in packets that correspond to the given rule to get forwarded without translation.

  • ALG support is currently limited to FTP, TFTP, and ICMP protocols. Also, although TCP SYN, TCP FIN and TCP RST are not part of ALG traffic, they are processed as part of ALG traffic.

  • Dynamically created NAT flows age out after a period of inactivity. The number of NAT flows whose activity can be tracked is limited to 240.

  • Port channel is not supported in NAT configuration.

  • NAT does not support translation of fragmented packets.

  • Explicit deny access control entry (ACE) in NAT ACL is not supported. Only explicit permit ACE is supported.

  • NAT and PBR share the same TCAM space and they cannot co-exist.

  • NAT configuration must be done without using route maps because route mapped NAT is not supported.

  • NAT is not supported for multicast packets.

Performance and Scale Numbers for NAT

The maximum number of bidirectional NAT flows supported in hardware is limited to 240.

Address Only Translation


Note

Using Address Only Translation optimizes the handling of flows and enhances the scale of the NAT feature.

You can use Address only Translation (AOT) functionality in situations that require only the address fields to be translated and not the transport ports. In such settings, enabling AOT functionality significantly increases the number of flows that can be translated and forwarded in the hardware at line-rate. This improvement is brought about by optimizing the usage of various hardware resources associated with translation and forwarding.

A typical NAT focused resource allocation scheme sets aside 512 TCAM entries for performing hardware translation. This places a strict upper limit on the number of flows that can be translated and forwarded at line-rate. Under AOT scheme, the usage of TCAM resource is highly optimized thereby enabling the accommodation of more number of flows in the TCAM tables and this provides a significant improvement in the hardware translation and forwarding scale.

AOT can be very effective in situations where majority of the flows are destined to a single or a small set of destinations. Under such favorable conditions, AOT can potentially enable line-rate translation and forwarding of all the flows originating from one or more given end-points. AOT functionality is disabled by default. It can be enabled using the no ip nat create flow-entries command. The existing dynamic flow can be cleared using the clear ip nat translation command. The AOT feature can be disabled using the ip nat create flow-entries command.

Restrictions for Address Only Translation

  • AOT feature is expected to function correctly only in translation scenarios corresponding to simple inside static and inside dynamic rules. The simple static rule must be of the type ip nat inside source static local-ip global-ip , and the dynamic rule must be of the type ip nat inside source list access-list pool name .

  • When AOT is enabled, the show ip nat translation command will not give visibility into all the NAT flows being translated and forwarded.

Configuring NAT

The tasks described in this section will help you configure NAT. Based on the desired configuration, you may need to configure more than one task.

Configuring Static Translation of Inside Source Addresses

Configure static translation of inside source address to allow one-to-one mapping between an inside local address and an inside global address. Static translation is useful when a host on the inside must be accessible by a fixed address from the outside.

Procedure

  Command or Action Purpose

Step 1

enable

Example:

Switch> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Switch# configure terminal

Enters global configuration mode.

Step 3

Use any of the following three commands depending on the requirement:

  • ip nat inside source static local-ip global-ip 
    Switch(config)# ip nat inside source static 10.10.10.1 172.16.131.
  • ip nat inside source static protocol local-ip port global-ip port 
    Switch(config)# ip nat inside source static tcp 10.10.10.1 1234 172.16.131.1 5467
  • ip nat inside source static network local-ip global-ip {prefix_len len | subnet subnet-mask} 
    Switch(config)# ip nat inside source static network 10.10.10.1 172.16.131.1 prefix_len 24

Establishes static translation between an inside local address and an inside global address.

Establishes a static port translation between an inside local address and an inside global address.

Establishes a static translation between an inside local address and an inside global address. You can specify a range of subnets to be translated to the inside global address, wherein the host portion of the IP address gets translated and the network portion of the IP remains the same.

Step 4

interface type number

Example:

Switch(config)# interface GigabitEthernet 1/1

Specifies an interface and enters interface configuration mode.

Step 5

ip address ip-address mask [secondary]

Example:

Switch(config-if)# ip address 10.114.11.39 255.255.255.0

Sets a primary IP address for an interface.

Step 6

ip nat inside

Example:

Switch(config-if)# ip nat inside

Connects the interface to the inside network, which is subject to NAT.

Step 7

exit

Example:

Switch(config-if)# exit

Exits interface configuration mode and returns to global configuration mode.

Step 8

interface type number

Example:

Switch(config)# interface GigabitEthernet 1/2

Specifies a different interface and enters interface configuration mode.

Step 9

ip address ip-address mask [secondary]

Example:

Switch(config-if)# ip address 172.31.232.182 255.255.255.240

Sets a primary IP address for an interface.

Step 10

ip nat outside

Example:

Switch(config-if)# ip nat outside

Connects the interface to the outside network.

Step 11

end

Example:

Switch(config-if)# end

Exits interface configuration mode and returns to privileged EXEC mode.

Configuring Dynamic Translation of Inside Source Addresses

Dynamic translation establishes a mapping between an inside local address and a pool of global addresses dynamically. Dynamic translation can be enabled by configuring a dynamic NAT rule and the mapping is established based on the result of the evaluation of the configured rule at run-time. You can employ an ACL to specify the inside local address and the inside global address can be specified through an address pool or an interface.

Dynamic translation is useful when multiple users on a private network need to access the Internet. The dynamically configured pool IP address may be used as needed and is released for use by other users when access to the internet is no longer required.

Procedure

  Command or Action Purpose

Step 1

enable

Example:

Switch> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Switch# configure terminal

Enters global configuration mode.

Step 3

ip nat pool name start-ip end-ip netmask netmask | prefix-length prefix-length

Example:

Switch(config)# ip nat pool net-208 172.16.233.208 172.16.233.223 prefix-length 28

Defines a pool of global addresses to be allocated as needed.

Step 4

access-list access-list-number permit source [source-wildcard]

Example:

Switch(config)# access-list 1 permit 192.168.34.0 0.0.0.255

Defines a standard access list permitting those addresses that are to be translated.

Step 5

ip nat inside source list access-list-number pool name

Example:

Switch(config)# ip nat inside source list 1 pool net-208

Establishes dynamic source translation, specifying the access list defined in Step 4.

Step 6

interface type number

Example:

Switch(config)# interface GigabitEthernet 1/1

Specifies an interface and enters interface configuration mode.

Step 7

ip address ip-address mask

Example:

Switch(config-if)# ip address 10.114.11.39 255.255.255.0

Sets a primary IP address for the interface.

Step 8

ip nat inside

Example:

Switch(config-if)# ip nat inside

Connects the interface to the inside network, which is subject to NAT.

Step 9

exit

Example:

Switch(config-if)#exit

Exits the interface configuration mode and returns to global configuration mode.

Step 10

interface type number

Example:

Switch(config)# interface GigabitEthernet 1/2

Specifies an interface and enters interface configuration mode.

Step 11

ip address ip-address mask

Example:

Switch(config-if)# ip address 172.16.232.182 255.255.255.240

Sets a primary IP address for the interface.

Step 12

ip nat outside

Example:

Switch(config-if)# ip nat outside

Connects the interface to the outside network.

Step 13

end

Example:

Switch(config-if)# end

Exits interface configuration mode and returns to privileged EXEC mode.

Configuring PAT

Perform this task to allow your internal users access to the Internet and conserve addresses in the inside global address pool using overloading of global addresses.

Procedure

  Command or Action Purpose

Step 1

enable

Example:

Switch> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Switch# configure terminal

Enters global configuration mode.

Step 3

ip nat pool name start-ip end-ip netmask netmask | prefix-length prefix-length

Example:

Switch(config)# ip nat pool net-208 192.168.202.129 192.168.202.158 netmask 255.255.255.224

Defines a pool of global addresses to be allocated as needed.

Step 4

access-list access-list-number permit source [source-wildcard]

Example:

Switch(config)# access-list 1 permit 192.168.201.30 0.0.0.255

Defines a standard access list permitting those addresses that are to be translated.

The access list must permit only those addresses that are to be translated. (Remember that there is an implicit “deny all” at the end of each access list.) Use of an access list that is too permissive can lead to unpredictable results.

Step 5

ip nat inside source list access-list-number pool name overload

Example:

Switch(config)# ip nat inside source list 1 pool net-208 overload

Establishes dynamic source translation with overloading, specifying the access list defined in Step 4.

Step 6

interface type number

Example:

Switch(config)# interface GigabitEthernet 1/1

Specifies an interface and enters interface configuration mode.

Step 7

ip address ip-address mask [secondary]

Example:

Switch(config-if)# ip address 192.168.201.1 255.255.255.240

Sets a primary IP address for an interface.

Step 8

ip nat inside

Example:

Switch(config-if)# ip nat inside

Connects the interface to the inside network, which is subject to NAT.

Step 9

exit

Example:

Switch(config-if)# exit

Exits interface configuration mode and returns to global configuration mode.

Step 10

interface type number

Example:

Switch(config)# interface GigabitEthernet 1/2

Specifies a different interface and enters interface configuration mode.

Step 11

ip address ip-address mask [secondary]

Example:

Switch(config-if)#  ip address 192.168.201.29 255.255.255.240

Sets a primary IP address for an interface.

Step 12

ip nat outside

Example:

Switch(config-if)# ip nat outside

Connects the interface to the outside network.

Step 13

end

Example:

Switch(config-if)# end

Exits interface configuration mode and returns to privileged EXEC mode.

Configuring NAT of External IP Addresses Only

By default, NAT translates the addresses embedded in the packet pay-load as explained in the section Using Application-Level Gateways with NAT. There might be situations where the translation of the embedded address is not desirable and in such cases, NAT can be configured to translate the external IP address only.

Procedure

  Command or Action Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

ip nat inside source {list {access-list-number | access-list-name } pool pool-name [overload ] | static network local-ip global-ip [no-payload ]}

Example:

Device(config)# ip nat inside source static network 10.1.1.1 192.168.251.0/24 no-payload

Disables the network packet translation on the inside host device.

Step 4

ip nat inside source {list {access-list-number | access-list-name } pool pool-name [overload ] | static {tcp | udp } local-ip local-port global-ip global-port [no-payload ]}

Example:

Device(config)# ip nat inside source static tcp 10.1.1.1 2000 192.168.1.1 2000 no-payload

Disables port packet translation on the inside host device.

Step 5

ip nat inside source {list {access-list-number | access-list-name} pool pool-name [overload ] | static [network ] local-network-mask global-network-mask [no-payload ]}

Example:

Device(config)# ip nat inside source static 10.1.1.1 192.168.1.1 no-payload

Disables packet translation on the inside host device.

Step 6

ip nat outside source {list {access-list-number | access-list-name } pool pool-name | static local-ip global-ip [no-payload ]}

Example:

Device(config)# ip nat outside source static 10.1.1.1 192.168.1.1 no-payload

Disables packet translation on the outside host device.

Step 7

ip nat outside source {list {access-list-number | access-list-name } pool pool-name | static {tcp | udp } local-ip local-port global-ip global-port [no-payload ]}

Example:

Device(config)# ip nat outside source static tcp 10.1.1.1 20000 192.168.1.1 20000 no-payload

Disables port packet translation on the outside host device.

Step 8

ip nat outside source {list {access-list-number | access-list-name } pool pool-name | static [network ] local-network-mask global-network-mask [no-payload ]}

Example:

Device(config)# ip nat outside source static network 10.1.1.1 192.168.251.0/24 no-payload

Disables network packet translation on the outside host device.

Step 9

exit

Example:

Device(config)# exit

Exits global configuration mode and returns to privileged EXEC mode.

Step 10

show ip nat translations [verbose ]

Example:

Device# show ip nat translations

Displays active NAT.

Configuring Translation of Overlapping Networks

Configure static translation of overlapping networks if your IP addresses in the stub network are legitimate IP addresses belonging to another network and you want to communicate with those hosts or routers using static translation.


Note

For a successful NAT outside translation, the device should be configured with a route for the outside local address. You can configure the route either manually or using the add-route option associated with ip nat outside source {static | list} command. We recommend that you use the add-route option to enable automatic creation of the route.

Procedure

  Command or Action Purpose

Step 1

enable

Example:

Switch> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Switch# configure terminal

Step 3

ip nat inside source static local-ip global-ip

Example:

Switch(config)# ip nat inside source static 10.1.1.1 203.0.113.2

Establishes static translation between an inside local address and an inside global address.

Step 4

ip nat outside source static local-ip global-ip

Example:

Switch(config)# ip nat outside source static 172.16.0.3 10.1.1.3

Establishes static translation between an outside local address and an outside global address.

Step 5

interface type number

Example:

Switch(config)# interface GigabitEthernet 1/1

Specifies an interface and enters interface configuration mode.

Step 6

ip address ip-address mask

Example:

Switch(config-if)# ip address 10.114.11.39 255.255.255.0

Sets a primary IP address for an interface.

Step 7

ip nat inside

Example:

Switch(config-if)# ip nat inside

Marks the interface as connected to the inside.

Step 8

exit

Example:

Switch(config-if)# exit

Exits interface configuration mode and returns to global configuration mode.

Step 9

interface type number

Example:

Switch(config)# interface GigabitEthernet 1/2

Specifies a different interface and enters interface configuration mode.

Step 10

ip address ip-address mask

Example:

Switch(config-if)#  ip address 172.16.232.182 255.255.255.240

Sets a primary IP address for an interface.

Step 11

ip nat outside

Example:

Switch(config-if)# ip nat outside

Marks the interface as connected to the outside.

Step 12

end

Example:

Switch(config-if)# end

Exits interface configuration mode and returns to privileged EXEC mode.

Configuring Address Translation Timeouts

You can configure address translation timeouts based on your NAT configuration.

By default, dynamically created translation entries time-out after a period of inactivity to enable the efficient use of various resources. You can change the default values on timeouts, if necessary. The following are the default time-out configurations associated with major translation types:

  • Established TCP sessions: 24 hours

  • UDP flow: 5 minutes

  • ICMP flow: 1 minute

The default timeout values are adequate to address the timeout requirements in most of the deployment scenarios. However, these values can be adjusted/fine-tuned as appropriate. It is recommended not to configure very small timeout values (less than 60 seconds) as it could result in high CPU usage. Refer the x section for more information.

Based on your configuration, you can change the timeouts described in this section.

  • If you need to quickly free your global IP address for a dynamic configuration, configure a shorter timeout than the default timeout, by using the ip nat translation timeout command. However, the configured timeout should be longer than the other timeouts configured using commands specified in the following steps.

  • If a TCP session is not properly closed by a finish (FIN) packet from both sides or during a reset, change the default TCP timeout by using the ip nat translation tcp-timeout command.

Procedure

  Command or Action Purpose

Step 1

enable

Example:

Switch> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Switch# configure terminal

Enters global configuration mode.

Step 3

ip nat translation seconds

Example:

Switch(config)# ip nat translation 300

(Optional) Changes the amount of time after which NAT translations time out.

The default timeout is 24 hours, and it applies to the aging time for half-entries.

Step 4

ip nat translation udp-timeout seconds

Example:

Switch(config)# ip nat translation udp-timeout 300

(Optional) Changes the UDP timeout value.

Step 5

ip nat translation tcp-timeout seconds

Example:

Switch(config)# ip nat translation tcp-timeout 2500

(Optional) Changes the TCP timeout value.

The default is 24 hours.

Step 6

ip nat translation finrst-timeout seconds

Example:

Switch(config)# ip nat translation finrst-timeout 45

(Optional) Changes the finish and reset timeout value.

finrst-timeout—The aging time after a TCP session receives both finish-in (FIN-IN) and finish-out (FIN-OUT) requests or after the reset of a TCP session.

Step 7

ip nat translation icmp-timeout seconds

Example:

Switch(config)# ip nat translation icmp-timeout 45

(Optional) Changes the ICMP timeout value.

Step 8

ip nat translation syn-timeout seconds

Example:

Switch(config)# ip nat translation syn-timeout 45

(Optional) Changes the synchronous (SYN) timeout value.

The synchronous timeout or the aging time is used only when a SYN request is received on a TCP session. When a synchronous acknowledgment (SYNACK) request is received, the timeout changes to TCP timeout.

Step 9

end

Example:

Switch(config-if)# end

Exits interface configuration mode and returns to privileged EXEC mode.

Configuring Network Address Translation on Layer 3 Port Channel

Procedure

Step 1

enable

Example:

Switch> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Switch# configure terminal

Enters global configuration mode.

Step 3

interface port-channel port-channel-number

Example:

Switch(config)# interface port-channel 10

Enters port-channel interface mode.

Step 4

ip address ip-address mask [secondary]

Example:

Switch(config)# ip address 10.114.11.39 255.255.255.0

Sets a primary IP address for an interface.

Step 5

ip nat inside

Example:

Switch(config)# ip nat inside

Connects the interface to the inside network, which is subject to NAT.

Step 6

interface port channel port-channel-number

Example:

Switch(config)# interface port-channel 11

Enters port-channel interface mode.

Step 7

ip address ip-address mask [secondary]

Example:

Switch(config)# ip address 172.31.232.182 255.255.255.240

Sets a primary IP address for an interface.

Step 8

ip nat outside

Example:

Switch(config)# ip nat outside

Connects the interface to the outside network.

Step 9

end

Example:

Switch(config)# end

Exits interface configuration mode and returns to privileged EXEC mode.

Using Application-Level Gateways with NAT

NAT performs translation services on any TCP/UDP traffic that does not carry source and destination IP addresses in the application data stream. Protocols that do not carry the source and destination IP addresses include the following:

  • HTTP

  • TFTP

  • Telnet

  • Archie

  • Finger

  • Network Time Protocol (NTP)

  • Network File System (NFS)

  • Remote login (rlogin)

  • Remote shell (rsh)

  • Remote copy (rcp)

NAT Application-Level Gateway (ALG) enables certain applications that carry address/port information in their payloads to function correctly across NAT domains. In addition to the usual translation of address/ports in the packet headers, ALGs take care of translating the address/ports present in the payload and setting up temporary mappings.

Best Practices for NAT Configuration

  • In cases where both static and dynamic rules are configured, ensure that the local addresses specified in the rules do not overlap. If such an overlap is possible, then the ACL associated with the dynamic rule should exclude the corresponding addresses used by the static rule. Similarly, there must not be any overlap between the global addresses as this could lead to undesired behavior.

  • Do not employ loose filtering such as permit ip any any in an ACL associated with NAT rule as this could result in unwanted packets being translated.

  • Do not share an address pool across multiple NAT rules.

  • Do not define the same inside global address in Static NAT and Dynamic Pool. This action can lead to undesirable results.

  • Exercise caution while modifying the default timeout values associated with NAT. Small timeout values could result in high CPU usage.

  • Exercise caution while manually clearing the translation entries as this could result in the disruption of application sessions.

  • ALG packets traversing a NAT enabled interface will get punted to CPU, regardless of the packets being translated or not. Therefore, it is recommended to use dedicated interface(s) just for NAT traffic. For all other types of traffic that does not require NAT translation, use a different interface(s).

Configuration Examples for Network Address Translation

Configuring Static Translation of Inside Source Addresses

The following example shows how inside hosts addressed from the 10.114.11.0 network are translated to the globally unique 172.31.233.208/28 network. Further, packets from outside hosts that are addressed from the 10.114.11.0 network (the true 10.114.11.0 network) are translated to appear from the 10.0.1.0/24 network. 

ip nat pool net-208 172.31.233.208 172.31.233.223 prefix-length 28
ip nat pool net-10 10.0.1.0 10.0.1.255 prefix-length 24
ip nat inside source list 1 pool net-208
ip nat outside source list 1 pool net-10
!
interface gigabitethernet 1/1
 ip address 172.31.232.182 255.255.255.240
 ip nat outside
!
interface gigabitethernet 1/2
 ip address 10.114.11.39 255.255.255.0
 ip nat inside
!
access-list 1 permit 10.114.11.0 0.0.0.255

The following example shows a static VRF aware NAT configuration to translate overlapping local addresses:

ip nat inside source static 192.168.121.33 10.2.2.1 vrf vrf1
ip nat inside source static 192.168.121.33.10.2.2.2 vrf vrf2

Configuring Dynamic Translation of Inside Source Addresses

The following example shows how inside hosts addressed from either the 192.168.1.0 or the 192.168.2.0 network are translated to the globally unique 172.31.233.208/28 network: 

ip nat pool net-208 172.31.233.208 172.31.233.223 prefix-length 9
ip nat inside source list 1 pool net-208
!
interface gigabitethernet 1/1
 ip address 172.31.232.182 255.255.255.240
 ip nat outside
!
interface gigabitethernet 1/2
 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 following example shows a dynamic VRF aware NAT configuration to translate overlapping local addresses:

ip nat inside source list 1 interface gigabitethernet 1/1 vrf vrf1 overload
!
ip route vrf vrf1 0.0.0.0 0.0.0.0 192.168.1.1
!
access-list 1 permit 10.1.1.1.0 0.0.0.255
!
ip nat inside source list 1 interface gigabitethernet 1/2 vrf vrf1 overload
!
ip route vrf vrf1 0.0.0.0 0.0.0.0 172.16.1.1 global
access-list 1 permit 10.1.1.0 0.0.0.255
!

Troubleshooting NAT

This section explains the basic steps to troubleshoot and verify NAT.

  • Clearly define what NAT is supposed to achieve.

  • Verify that correct translation table exists using the show ip nat translation command.

  • Verify that timer values are correctly configured using the show ip nat translation verbose command.

  • Check the ACL values for NAT using the show ip access-list command.

  • Check the overall NAT configuration using the show ip nat statistics command.

  • Use the clear ip nat translation command to clear the NAT translational table entires before the timer expires.

  • Use debug nat ip and debug nat ip detailed commands to debug NAT configuration.

For further information on Troubleshooting NAT, see Verifying NAT Operation and Basic NAT Troubleshooting on Cisco.com..

Feature History for Network Address Translation

This table provides release and related information for the features explained in this module.

These features are available in all the releases later to the one they were introduced in, unless noted otherwise.

Release

Feature

Feature Description

Cisco IOS XE Cupertino 17.7.1

Layer 3 Network Address Translation for Cisco Catalyst IE9300 Rugged Series Switches

NAT enables private IP networks that use unregistered IP address to connect to the internet. NAT operates on a device, usually connecting two networks together, and translates the private addresses in the internal network into global routable addresses, before packets are forwarded onto another network.

Support for this feature was introduced for the following switch models:

  • IE-9310-26S2C-A

  • IE-9320-26S2C-A