Network Address Translation (NAT)

What is NAT?

NAT stands for “Network Address Translation” it allows a host that does not have a valid registered IP address communicates with other hosts through the internet. NAT uses a valid registered IP address to represent the non-registered IP address. Non-registered IP address used inside a network that is known as “Private Address”. NAT operates on a router, usually connecting two networks together, and translates the non-register private addresses into registered legal public addresses before packets are forward to another network. NAT offers the dual functions of security and address conservation and is typically implemented in remote-access environments. It allows a single device, such as a router, to act as an agent between the public network or internet and private network or local network and a single unique IP address is required to represent an entire group of computers to anything outside their network.

NAT Addresses Types

Inside Addresses - It refers to the addresses which must be translated.

Outside Addresses - It refers to the addresses which are not in control of an organization.

Example for NAT

Inside Local Address

Inside the local network assigned an IP address to the host is known as Inside local address. These IP addresses are the private IP address and these inside hosts can be accessed from the inside network.

Inside Global Address

One or more inside local IP address represents to the outside world is known as inside global address. These inside hosts can able to access from the outside network.

Outside Local Address

After translation, the outside local address is the actual IP address of the destination host in the local network.

Outside Global Address

Before translation, the outside global address is the IP address of the outside destination host, where, the outside host can able access from the outside network.

Range of IP Address

1. Class A – 10.X.X.X (10.0.0.0 TO 10.255.255.255)

2. Class B – 172.16.X.X (172.16.0.0 to 172.31.255.255)

3. Class C - 192.168.X.X (192.168.0.0 to 192.168.255.255)

NAT Types

There are three types of NAT Configuration

1. Static NAT
2. Dynamic NAT
3. Overloading NAT

Static NAT

A single private IP address is map with a single public address that is one to one mapping between the local and global addresses. This technique is generally use for web hosting. In this, users can also able to configure static address translation at a port level, and also use a remainder for other translations. This can be possible only if you were performing Port Address Translation (PAT). For example, if there are 1000 network devices are available which need access to the internet, then the organization needs to buy 1000 public IP addresses that will be very costly to buy.

Dynamic NAT

A private IP address is translate into a public IP address from the pool of IP address or interface IP address that is the users can establish dynamic mapping between the local and global addresses. Such that sets up a pool of possible inside global addresses and defines criteria for the set of inside local IP addresses whose traffic should be translated with NAT. For example, if there is a pool of 2 public IP addresses then only 2 private IP addresses can be translated at a given time. If 3rd private IP address wants to access the Internet then the packet will be dropped because of this the organization has to buy many global IP addresses to make a pool is also very costly to buy.

Overloading NAT with PAT

NAT overload uses PAT (Port Address Translation) this is a translation table entry containing the IP address and source/destination port information, which is called PAT or overloading. In this, many private IP addresses can be translate into a single public IP address and port numbers are use to identify the traffic (i.e. which traffic belongs to which address). It supports lots of inside local IP addresses with only a few inside global or publicly registered IP addresses. Instead of just translating the IP address, it also translates the port number.

Benefits of NAT

• Conserves legally registered IP addresses
• Eliminates address renumbering when a network evolves.
• Provides privacy as the device IP address
• Sending and receiving the traffic, will be hidden.

Drawbacks of NAT

• Delays translation results in switching path
• when NAT is enable certain applications will not function
• Complicates tunneling protocols such as IPsec.

Application Layer Gateway (ALG)

ALG is an Application Layer Gateway (ALG). NAT performs translation service on any Transmission Control Protocol/User Datagram Protocol (TCP/UDP) traffic that does not carry source and/or destination IP addresses in the application data stream. These protocols include FTP, HTTP, SKINNY, H232, DNS, RAS, SIP, TFTP, telnet, Archie, finger, NTP, NFS, rlogin, rsh, rcp. Specific protocols that embed IP address information within the payload require the support of an Application Level Gateway (ALG).

NAT Software List

• Internet Connection Sharing (ICS) – NAT & DHCP implementation for Windows desktop operating systems
• IPFirewall (IPFW) - FreeBSD Native Packet Filter
• IPFilter includes Solaris (Open) and NetBSD for Unix operating systems
• Routing and Remote Access Service – Routing implementation for Windows server operating systems
• WinGate – Third party implementation for Windows operating systems
• NetFilter with IPTables and NFTables – Linux Packet Filter
• Native Packet Filter (NPF) – NetBSD
• Packet Filter (PF) – OpenBSD

Also learn about Integrated Services Digital Network (ISDN)