Download Transcript: Introducing NAT

Hello, I'm Brian Ferrill, and welcome to PACE-IT’s session on introducing network address
translation. Today, we're going to be talking about the purpose of network address translation
and then we're going to discuss how network address translation works. With that, let's go
ahead and begin this discussion.
Of course, we're going to begin by talking about the purpose of network address translation.
Network address translation, or NAT, solves a very serious problem of how to route nonroutable IP addresses. As a partial effort to conserve the IPv4 address space, the private IPv4
addressing spaces were developed. These address spaces were removed from the public IPv4
address space, and made non-routable across public IPv4 networks. This led to a problem;
being non-routable prevents that private IPv4 address from communicating with remote public
networks.
NAT very simply solves this problem. A router with NAT enabled, will translate a private IP
address into a routable, public IP address. When the response returns to the router, it passes
the response back to the device that requested it.
So, now that we've covered the purpose, let's talk about how network address translation works.
First off, we get to talk about the fact that there are two categories of NAT. First up is static NAT.
With static NAT, each private IP address is assigned to a specific, routable public IP address.
This relationship is kept and maintained by the NAT enabled router. When a device needs
access outside of the local network, the router translates the local IP address to the assigned
public IP address. When the response comes back, the router will translate the public IP
address back into a local one. Static NAT is not flexible and leads to some scalability issues. An
individual, routable IP address must be kept for every device that requires access outside of the
local network, so as the network grows, you need to increase the amount of public IP addresses
that are under your control. That gets kind of expensive and kind of complicated.
They developed dynamic NAT to resolve some of that issue. With dynamic NAT, the NAT
enabled router dynamically assigns a routable IP address to devices from a pool of available IP
addresses. When a device needs access outside of the local network, the router performs the
NAT function, only the public IP address comes from a reusable pool of public IP addresses.
That private IP address is assigned the public IP address from the pool and, once outside
access is stopped, the routable IP address goes back into the pool to be reused. As initially
designed, dynamic NAT was more flexible than static NAT, but it still led to some scalability
issues. As more network traffic required access to outside networks, the pool of available public
IP addresses needs to increase, or outside access cannot be achieved. But thankfully, there is a
solution to this.
That solution is called port address translation, or in Cisco terms, that would be NAT with PAT.
PAT is a type of dynamic NAT that was developed to increase the scalability of network address
translation. When a local network device requires access to a public network, the NAT enabled
router dynamically assigns the public IP address to the device with the addition of dynamically
assigning a port number to the end of the public IP address. The router tracks the IP addresses
and port numbers to ensure that network traffic is routed to and from the proper devices. PAT
still requires a pool of public IP addresses, but the pool may only contain one public IP address,
or it may contain several for a large, private network.
This is the preferred method of implementing network address translation for two reasons. First
off, there's less public IP addresses that are required and, second, it makes it easier for an
administrator to maintain.
Now, let's talk about NAT terminology, specifically, about the types of addresses. We begin with
the inside local address, which is a private IP address on the local network. It is the private IP
address assigned to a specific device. Then there's the inside global address, a public address
referencing an inside device. The inside global address is the public IP address assigned to the
inside device by the NAT enabled router, allowing access outside of the network. Then there's
the outside global address, which is a public IP address referencing an outside device. It is the
public IP address assigned to a device outside of the local network. Then there's the outside
local address, which is a private IP address assigned to an outside device. This is the private IP
address assigned to the outside device by the NAT enabled router on the interior of the local
network, so that the inside device can communicate correctly with the outside device.
Now, that concludes this session on introducing network address translation. We talked about
the purpose of network address translation. Then we talked about how network address
translation works. On behalf of PACE-IT, thank you for watching this session, and I look forward
to doing another one.