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DEMO: PAT (using interface)

Port Address Translation (PAT) can be configured to translate all internal traffic to a single public IP address assigned to a router's external interface, which reflects how most real-world networks connect to an ISP. This demonstration walks through the full configuration and uses Wireshark to verify the address translation in action.

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About this video

Most networks connected to an ISP receive only a single public IP address, which means traditional NAT pools are not an option. Instead, PAT can be configured to translate all outbound traffic from internal hosts to the single IP address assigned to the router's external interface, using port numbers to track individual sessions and allow multiple devices to share that one address simultaneously. This approach is one of the most common NAT configurations in production environments, from home routers to small and mid-sized business networks. The configuration involves creating an access list that identifies the internal address range, then applying the ip nat inside source command referencing that list with the interface keyword pointed at the external interface and the overload keyword to enable PAT. Inside and outside NAT roles are assigned to the appropriate router interfaces to complete the setup. A before-and-after Wireshark capture demonstrates the practical effect of the configuration: traffic originating from an internal switch address is retransmitted with the router's external interface IP as its source, confirming that translation is functioning as expected. This method is particularly significant because it directly reflects real-world ISP connectivity scenarios where a single routable address must serve an entire internal network.

What you'll learn

What's covered

PAT to Specific Interface

Aligned to

Cisco CCNA
4.1 Configure and verify inside source NAT using static and pools
1.7 Describe private IPv4 addressing

Key terms

Network Address Translation
NAT
A method that maps private IP addresses to a public IP address, allowing multiple devices to share a single public IP.
Port Address Translation
PAT
A form of NAT that maps multiple devices on a private network to a single public IP address by differentiating each connection using unique source port numbers. PAT is the most common NAT implementation and is what enables an entire home or office network to share a single public IP address.
Access Control List
ACL
A set of rules that defines which users or systems are granted or denied access to a resource.
Router
A network device that forwards data packets between networks based on IP addresses.
IP Address
A numerical label assigned to each device connected to a network that uses the Internet Protocol.
Port
A logical endpoint for communication in a network, identified by a number that specifies a particular service or application.

Transcript

Quite often on our networks we only have one IP address, a public IP address. We have some sort of external router or firewall that's given an IP address, and that's what we have to work with. Therefore we don't have a range of IP addresses that we can translate to, but we have one IP address, and that is the IP address of the interface that we have to do all of our translation to. Let's take a look at what that looks like when we do PAT to a specific interface.

The setup

This is my network right here. This time we're going to use router 2 to do the network address translation. What we're going to do is generate traffic from this switch right here through this router 2, which is going to be translated into another IP address, specifically the IP address of this interface on router 2, in which case then it will generate to this demo laptop. We're going to use Wireshark to confirm that we've changed the address.

Before we do this, let's go ahead and do a ping from this switch to the demo laptop and check out the IP address. I'm going to do a ping of that laptop, and we pull up Wireshark, and we can see here it came from 10.2.0.5. That makes sense, because that's the IP address of that switch.

The NAT configuration

Now let's get in there and do our NAT configuration. What I'm going to do is still specify the range of addresses that we're going to translate, so these are the local addresses. If I pull up this network diagram, that's all of the addresses right here in this network. I'm going to copy that, and I've got to get into the global configuration mode. I will create this access list one, in which I'm defining that network that we just covered right there.

Then I'm going to use this — this is the same ip nat inside source that we're used to. It's using list one, so this is the local addresses that it's going to be translating from, and it's going to translate to whatever interface s010 is going to be. It's going to be doing PAT, and we see that because of the overload statement.

Let's get this into our network here. I'm going to paste that into there. I'm going to of course have to define what the inside network is, so I'll define that, and what is the outside: s010 is going to be the outside, so I'll do that. It's that same error message we've been receiving, so that's expected.

Confirming the translation

Now let's go ahead and generate the traffic again. We'll generate traffic, and it should pop up here, and it's coming from 10.240.2.1. So the IP address did change, and it changed to this interface right here, which is s010 on router 2. We were successful; we are now using this interface.

This is a common setup, because once again, when you're connecting to your internet service provider, you may not have a range of addresses but are just given a single IP address, and so this is how a lot of places are set up.

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