NAT offers the dual functions of security and address conservation and is typically implemented in remote-access environments. Basically, NAT allows a single device, such as a router, to act as an agent between the Internet or public network and a local network or private network , which means that only a single unique IP address is required to represent an entire group of computers to anything outside their network.
In order to configure traditional NAT, you need to make at least one interface on a router NAT outside and another interface on the router NAT inside and a set of rules for translating the IP addresses in the packet headers and payloads if desired need to be configured. The main differences include the different traffic types supported in the implementations. Refer to Cisco Feature Navigator in order to use this tool.
The order in which the transactions are processed using NAT is based on whether a packet is going from the inside network to the outside network or from the outside network to the inside network. Inside to outside translation occurs after routing, and outside to inside translation occurs before routing. Using NAT, you can establish a virtual host on the inside network that coordinates load sharing among real hosts.
In addition to giving users more control over how NAT addresses are used, the Rate-Limiting NAT Translation feature can be used to limit the effects of viruses, worms, and denial-of-service attacks. Static route entry is configured in the next-hop router and redistributed within the routing network. When the inside global address is matched with the local interface, NAT installs an IP alias and an ARP entry, in which case the router will proxy-arp for these addresses.
If this behavior is not wanted, use the no-alias keyword. When a NAT pool is configured, the add-route option can be used for automatic route injection. As a result, 10, translations more than would generally be handled on a single router consume about 3 MB. Therefore, typical routing hardware has more than enough memory to support thousands of NAT translations. For The current session is not maintained when failure takes place. Encapsulation does not matter for NAT. There must be an inside and an outside for NAT to function.
This can be accomplished through the use of an access list describing the set of hosts or networks that require NAT. All sessions on the same host will be either translated or will pass through the router and not be translated. Access lists, extended access lists, and route maps can be used to define rules by which IP devices get translated. The network address and appropriate subnet mask should always be specified. The keyword any should not be used in place of the network address or subnet mask.
PAT overloading divides the available ports per global IP address into three ranges: , , and It attempts to assign the same port value of the original request, but if the original source port has already been used, it starts scanning from the beginning of the particular port range to find the first available port and assigns it to the conversation.
There is an exception for To define a pool, the configuration command is used:. The following example translates between inside hosts addressed from either the In the following example, the goal is to define a virtual address, connections to which are distributed among a set of real hosts. The pool defines the addresses of the real hosts. The access list defines the virtual address.
If a translation does not already exist, TCP packets from serial interface 0 the outside interface whose destination matches the access list are translated to an address from the pool. In practical use, the maximum number of configurable IP pools is limited by the amount of available DRAM in the particular router. Cisco recommends that you configure a pool size of Each pool should be no more than 16 bits.
In This has limited NAT to only have a maximum of pools. It also has the capability to map a single inside IP address to different Inside Global addresses based on the rule. IP address overlapping refers to a situation where two locations that want to interconnect are both using the same IP address scheme.
This is not an unusual occurrence; it often happens when companies merge or are acquired. Without special support, the two locations will not be able to connect and establish sessions. The overlapped IP address can be a public address assigned to another company, a private address assigned to another company, or can come from the range of private addresses as defined in RFC Private IP addresses are unroutable and require NAT translations to allow connections to the outside world.
The solution involves intercepting Domain Name System DNS name-query responses from the outside to the inside, setting up a translation for the outside address, and fixing up the DNS response before forwarding it to the inside host.
A DNS server is required to be involved on both sides of the NAT device to resolve users wanting to have connection between both networks. Static NAT translations have one-to-one mapping between local and global addresses. You can still have some computers on the stub domain that use dedicated IP addresses. You can create an access list of IP addresses that tells the router which computers on the network require NAT. All other IP addresses will pass through untranslated.
But since a typical entry in the address-translation table only takes about bytes, a router with 4 MB of DRAM could theoretically process 26, simultaneous translations, which is more than enough for most applications.
IANA has set aside specific ranges of IP addresses for use as non-routable, internal network addresses. These addresses are considered unregistered for more information check out RFC Address Allocation for Private Internets , which defines these address ranges. No company or agency can claim ownership of unregistered addresses or use them on public computers. Routers are designed to discard instead of forward unregistered addresses. What this means is that a packet from a computer with an unregistered address could reach a registered destination computer, but the reply would be discarded by the first router it came to.
Although each range is in a different class, your are not required to use any particular range for your internal network. It is a good practice, though, because it greatly diminishes the chance of an IP address conflict.
Implementing dynamic NAT automatically creates a firewall between your internal network and outside networks, or between your internal network and the Internet. NAT only allows connections that originate inside the stub domain.
Essentially, this means that a computer on an external network cannot connect to your computer unless your computer has initiated the contact. You can browse the Internet and connect to a site, and even download a file; but somebody else cannot latch onto your IP address and use it to connect to a port on your computer.
In specific circumstances, Static NAT, also called inbound mapping , allows external devices to initiate connections to computers on the stub domain. For instance, if you wish to go from an inside global address to a specific inside local address that is assigned to your Web server, Static NAT would enable the connection.
Some NAT routers provide for extensive filtering and traffic logging. Filtering allows your company to control what type of sites employees visit on the Web, preventing them from viewing questionable material. You can use traffic logging to create a log file of what sites are visited and generate various reports from it. NAT is sometimes confused with proxy servers , but there are definite differences between them. NAT is transparent to the source and to destination computers.
Neither one realizes that it is dealing with a third device. But a proxy server is not transparent. The source computer knows that it is making a request to the proxy server and must be configured to do so. The destination computer thinks that the proxy server IS the source computer, and deals with it directly.
Working at a higher layer makes proxy servers slower than NAT devices in most cases. A real benefit of NAT is apparent in network administration. For example, you can move your Web server or FTP server to another host computer without having to worry about broken links. Simply change the inbound mapping at the router to reflect the new host. You can also make changes to your internal network easily, because the only external IP address either belongs to the router or comes from a pool of global addresses.
You can choose a range of unregistered IP addresses for your stub domain and have the DHCP server dole them out as necessary. It also makes it much easier to scale up your network as your needs grow.
Instead, you can just increase the range of available IP addresses configured in DHCP to immediately have room for additional computers on your network.
As businesses rely more and more on the Internet, having multiple points of connection to the Internet is fast becoming an integral part of their network strategy. Multiple connections, known as multi-homing , reduces the chance of a potentially catastrophic shutdown if one of the connections should fail. In addition to maintaining a reliable connection, multi-homing allows a company to perform load-balancing by lowering the number of computers connecting to the Internet through any single connection.
Distributing the load through multiple connections optimizes the performance and can significantly decrease wait times. Multi-homing really makes a difference if one of the connections to an ISP fails.
As soon as the router assigned to connect to that ISP determines that the connection is down, it will reroute all data through one of the other routers. NAT can be used to facilitate scalable routing for multi-homed, multi-provider connectivity. For more on multi-homing, see Cisco: Enabling Enterprise Multihoming. Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. Mobile Newsletter chat dots. Mobile Newsletter chat avatar.
Mobile Newsletter chat subscribe. Computer Hardware. How Network Address Translation Works. By: Jeff Tyson. Network Address Translation helps improve security by reusing IP addresses. The NAT router translates traffic coming into and leaving the private network. See more pictures of computer networking. In network address translation, a network device, often a router or NAT firewall, assigns a computer or computers inside a private network a public address.
In this way, network address translation allows the single device to act as an intermediary or agent between the local, private network and the public network that is the internet. Before NAT forwards packets between the networks it connects, it translates the private internal network addresses into legal, globally unique addresses. NAT configurations can reveal just one IP address for an entire network to the outside world as part of this capability, effectively hiding the entire internal network and providing additional security.
Network address translation is typically implemented in remote-access environments, as it offers the dual functions of address conservation and enhanced security. To communicate with the internet, a networking system requires a unique IP address. This bit number identifies and locates the network device so a user can communicate with it. The IPV4 addressing scheme of past decades technically made billions of these unique addresses available, but not all could be assigned to devices for communication.
Instead, some were exempted and used for testing, broadcast, and certain reserved military purposes. While that left over 3 billion for communication, the proliferation of the internet has meant the addresses were near exhaustion.
The IPv6 addressing scheme was introduced as the solution to this weakness in the IPv4 addressing scheme. IPv6 recreates the addressing system so there are more options for allocating addresses, but it has taken several years to alter the networking system infrastructure and to implement. NAT was introduced by Cisco in the meantime and widely deployed.
Network address translation permits a single device, such as a NAT firewall or NAT router or other network address translation device, to act as an agent between the public network and private networks—the internet and any local networks.
This allows an entire group of devices to be represented by a single unique IP address when they do anything outside their network. They tell the receptionist they need to speak with you, and the receptionist a checks the instructions and knows you want the call forwarded, and b matches your extension with a list to send the information to the right place.
The caller never gets your private line. Network address translation works similarly. The request arrives at the public IP address and port, and the NAT instructions send it where it should go without revealing the private IP addresses of the destinations.
As a NAT network address translation example, an inside host may want to communicate with a destination network address translation web server address in the outside world. The NAT gateway router determines whether the packet meets the condition for translation by learning the source IP address of the packet and looking it up in the table.
It can locate authenticated hosts for the internal network translation purposes on its access control list ACL , and then complete the translation, producing an inside global IP address from the inside local IP address.
Finally, the NAT gateway router will route the packet to the destination after saving the translation in the NAT table. Referring back to the NAT table, the router can determine which translated IP address corresponds to which global address, translate it to the inside local address, and deliver the data packet to the host at their IP address.
The data packet is discarded if no match is found. Static network address translation SNAT. It is particularly useful when a device needs to be accessible from outside the network. Dynamic network address translation DNAT.
This form of NAT selects a target from a group of registered IP addresses and maps an unregistered IP address to the registered version. Reverse network address translation RNAT. RNAT allows users to connect to themselves using the internet or public network. Overloading network address translation NAT. In terms of port address translation vs network address translation, PAT is often most cost-effective when many users are connected to the internet through just one public IP address.
Overlapping network address translation NAT. Overlapping NAT can happen either when two organizations whose networks both use RFC IP addresses merge, or when registered IP addresses are assigned to multiple devices or otherwise in use on more than one internal network.
In both cases, the networks need to communicate, and the organization s use overlapping NAT to achieve this without readdressing all devices. The NAT router intercepts addresses, and maintains a table of them so that it can replace them with registered unique IP addresses. The network address translation router must both translate registered external IP addresses to those unique to the private network and translate internal IP addresses to registered unique addresses.
In the network address translation context, the internal network, commonly referred to as the stub domain, is usually a local area network LAN that uses IP addresses internally.
Most stub domain network traffic is local, remaining inside the internal network. A stub domain may include both unregistered and registered IP addresses. A traditional NAT configuration requires at least one interface on a router NAT outside ; another interface on the router NAT inside ; and a configured set of rules for translating the IP addresses in the packet headers and possibly payloads.
In this example of network address translation configuration, IT configures the NAT router as follows.
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