Configuring NAT For IP Address Conservation in Cisco

IP address conservation is the goal of Network Address Translation (NAT). It makes it possible for private IP networks to connect to the Internet using unregistered IP addresses. Before packets are routed to another network, NAT acts as a router, typically connecting two networks, and converts the private (not globally unique) addresses in the internal network into lawful addresses. NAT can be set up to only advertise one address to the outside world for the entire network as part of this functionality. Effectively concealing the entire internal network behind one address, boosts security. NAT is frequently used in contexts with remote access because it provides the combined benefits of security and address conservation.

Requirements for NAT Configuration:

• Access Lists: All access lists required for usage with the configuration tasks provided must be configured before beginning a configuration activity.
• NAT Prerequisites: Before configuring NAT on your network, make sure you understand which interfaces are set up for NAT and for what purposes. 

The following prerequisites will assist you in determining how to configure and operate NAT:

• Users exist through a variety of interfaces.
• The internet is accessible via several interfaces.

NAT Configuration Purpose:

• Allow internal users’ internet access.
• Allow internet access to internal devices like a mail server.
• Allow overlapping networks to communicate with one another.
• Allow networks with various address schemes to communicate with one another.
• TCP traffic is redirected to another TCP port or address.
• NAT should be used during a network transition.

Working of NAT:

In essence, NAT enables a single device, such as a router, to operate as an intermediary between a local network and the Internet (or public network). As a result, just a single distinctive IP address is needed to represent an entire group of machines to anyone outside their network. Traditional NAT requires the creation of at least two router interfaces, one for NAT outside and the other for NAT within, as well as the configuration of rules for translating the IP addresses in packet headers (and payloads, if required). You need at least one interface configured with NAT enabled together with the same set of rules as indicated above in order to configure Nat Virtual Interface (NVI).

Types of NAT:

Only two networks are typically connected when NAT is used on a router. The private (within local) addresses within the internal network are converted into public (inside global) addresses by NAT before any packets are sent to a different network. You have the option to set up NAT using this functionality so that it only advertises one address to the outside world for your entire network. By performing this translation, NAT efficiently conceals the internal network from the outside world, enhancing your security. NAT can take the following forms:

• One-to-one mapping between local and global addresses is possible using static address translation (static NAT).
• Unregistered IP addresses are translated into registered IP addresses from a pool of registered IP addresses using dynamic address translation (dynamic NAT).
• Overloading Uses different ports to map numerous unregistered IP addresses to a single registered IP address (many to one). Port Address Translation is another name for this technique (PAT). One single worldwide IP address can be used to overload thousands of users’ connections to the Internet.

Outside and Inside NAT Addresses:

Networks controlled by an organization that needs to be translated are referred to as within in the context of Network Address Translation (NAT). When NAT is set up, hosts on this network have addresses that fit in a single space (known as the local address space). Users outside the network perceive these hosts as being in a different area (known as the global address space). In a similar vein, networks that the stub network connects to but are not managed by an organization are referred to as outside. Additionally, hosts on external networks may be translated and so have both local and global addresses. The following definitions are used by NAT:

• An IP address that is given to a host on the inside network is known as an inside local address. Most likely, the address given by the Network Information Center (NIC) or service provider is an invalid IP address.
• Inside global address: A valid IP address assigned by the NIC or service provider that, to the outside world, corresponds to one or more inside local IP addresses
• The IP address of an external host as seen by an internal network is known as the “outside local address.” It is assigned from the address space that is internally routable, however, it might not always be a valid address.
• The IP address that a host’s owner assigns to it on an external network is known as the outside global address. The address is chosen from a network space or globally routable address.

Benefits of Configuring NAT:

• When businesses must access the Internet using their existing networks yet face IP address depletion, NAT enables them to do so. 
• Websites must get IP addresses that have been registered with the Network Information Center (NIC). The lack of Class B addresses becomes a severe problem if there are more than 254 clients present or anticipated. In order to solve these problems, Cisco IOS XE NAT converts thousands of internal addresses that are hidden to a set of accessible Class C addresses.
• Websites that already have IP addresses registered for users on an internal network might want to keep those addresses private online. This move makes it impossible for hackers to target customers directly. A level of security is established by hiding the clients’ addresses. 
• LAN administrators have the total ability to increase Class A addressing using Cisco IOS XE NAT. The Internet Assigned Numbers Authority’s reserve pool is used for the Class A addressing extension (RFC 1597). This organizational growth takes place without taking into account address changes at the LAN/Internet interface.
• NAT can be carried out selectively or dynamically using the Cisco IOS XE software. The network administrator can utilize a combination of RFC 1597, RFC 1918, and registered addresses because of this flexibility. NAT is made to be used on a variety of devices for IP address conservation and simplification. Furthermore, Cisco IOS XE NAT enables the choice of internal hosts that are open to NAT.
• NAT’s ability to be configured without requiring modifications to network hosts or devices is a key benefit. On a few other devices where NAT is set up, adjustments are needed.
• Multi-Tenant support for the NAT capability was added in Cisco IOS XE Denali 16.3 version. A Virtual Routing and Forwarding (VRF) instance’s configuration changes are not affected by multi-tenant support. The configuration modifications made to one Virtual Routing and Forwarding (VRF) instance do not stop the traffic moving via other VRFs in the network thanks to multi-tenant support.
• NAT is a feature that allows an organization’s IP network to appear to the outside world to be utilizing a different IP address space than it actually is. Thus, NAT enables a nonglobally routable organization to connect to the Internet by translating their addresses into a worldwide routable address space. For enterprises shifting service providers or voluntarily renumbering into classless interdomain routing (CIDR) blocks, NAT also allows for a gentle renumbering technique. RFC 1631 describes network address translation (NAT).

Uses of NAT:

The following circumstances can benefit from NAT:

• Connect to the internet if none of your hosts has a globally unique IP address. Private IP networks that employ unregistered IP addresses can connect to the Internet via Network Address Translation (NAT). NAT is set up on a device at the boundary between a stub domain (referred to as the inner network) and a public network such as the Internet (mentioned as the outside network). Before transmitting packets to the outside network, NAT converts internal local addresses to globally unique IP addresses. NAT is only useful as a connectivity solution when only a few hosts in a stub domain communicate outside the domain at the same time. Only a little portion of the domain’s IP addresses must be converted into globally unique IP addresses when external communication is required. Additionally, once these addresses are no longer needed, they can be utilized again.
• Replace the internal addresses. Instead of updating the internal addresses, which can require a lot of work, you can use NAT to translate them.
• For TCP traffic’s fundamental load-sharing. TCP Load Distribution allows you to map a single global IP address to many local IP addresses.