Lesson 54 - Network Address Translation Part 4

It's time to put our theory into practice. We're going to use a single topology and try out different scenarios. They are not related to one another but my intention is to show you the variety of different methods in use. Keep in mind that they are just the few fundamental types of translations but there are more options available and they can become a bit more complex than the ones presented here.

Pic. 1 - Network Topology.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

R2 is playing a role of ISP's router. For the purpose of this presentation I'm not going to use real IP public addresses. I'm going to use 172.x.x.x range and pretend they are public ones.

Addresses used in this lesson are going to be as follows:
Private (LAN) IP = 192.168.1.0/24 and 192.168.2.0/24
Public (Internet) IP = 172.16.1.0/30 (connection to ISP)
Public (Internet) IP = 172.30.1.1 (Internet host used to check NAT)

All these addresses are simulated using Loopback interfaces.

R1 Initial Configuration:

!

interface Loopback1
 ip address 192.168.1.1 255.255.255.0
!

interface Loopback2
 ip address 192.168.2.1 255.255.255.0
!

interface FastEthernet1/0
 ip address 172.16.1.1 255.255.255.252
!

ip http server

!

ip route 0.0.0.0 0.0.0.0 172.16.1.2

!

R2 Initial Configuration:

!

interface Loopback0
 ip address 172.30.1.1 255.255.255.0
!

interface FastEthernet1/0
 ip address 172.16.1.2 255.255.255.252

!

ip http server

!

ip route 172.20.1.0 255.255.255.252 172.16.1.1

!

Scenario 1 - Static NAT
In this scenario I'm going to configure a static (one-to-one) NAT. This type of configuration allows me to present my public services to the Internet clients. Let's pretend that our 192.168.1.1 address is a Web server and is connected to R1 (loopback1 simulates this server). Our ISP gave us two public IP addresses (we pretend they are public anyway).

Inside Global Address Pool:

• 172.20.1.1/30
• 172.20.1.2/30

Our web server (192.168.1.1) will be seen on the Internet as 172.20.1.1. Below is the configuration to accomplish that.

R1 NAT Configuration:

!

interface Loopback1
 ip address 192.168.1.1 255.255.255.0
 ip nat inside

!

interface FastEthernet1/0
 ip address 172.16.1.1 255.255.255.252
 ip nat outside

!

ip nat inside source static 192.168.1.1 172.20.1.1

!

Note!
When you assign inside or outside role on the interface, the router is going to add 'ip virtual-reassembly' (dependant on the IOS version you use). At the CCNA level you can ignore the meaning of this command.

Verification:

Pic. 2 - NAT Table.

Note!
Static entry is created allowing transmission towards 192.168.1.1 which is seen as 172.20.1.1 on the Internet. Since we have not specified any layer 4 protocols (tcp/udp) and their ports, all ports are open in R1 allowing access to all ports on the web server. If you wanted to open only TCP port 80, you could use this command instead (more likely in real life scenarios):

!

ip nat inside source static tcp 192.168.1.1 80 172.20.1.1 80

!

Pic. 3 - Accessing Web Service.

Connection succeeds!

Scenario 2 - Dynamic NAT using IP Address Pool.
In this scenario we have the same pool of pretend-to-be-public IP addresses from scenario 1. This scenario will dynamically pick the first available IP address and use it for the host that wishes to send packets to the Internet. Since we only have two public IP addresses, only two hosts can send traffic at the time. Adding the 'overload' keyword will include the port translation and more hosts can reuse these two public addresses (172.20.1.1-2/30).

R1 NAT Configuration:

!

interface Loopback1
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
!

interface Loopback2
 ip address 192.168.2.1 255.255.255.0
 ip nat inside

!

interface FastEthernet1/0
 ip address 172.16.1.1 255.255.255.252
 ip nat outside
!

!

! The below command defines public IP addresses in the pool.

!

ip nat pool ISP_POOL 172.20.1.1 172.20.1.2 prefix-length 30
!

!

! The ACL1 matches on both subnets, the candidates for translation

! IP addresses that match the ACL1's statements, will be NATed.

!

access-list 1 permit 192.168.1.0 0.0.0.255
access-list 1 permit 192.168.2.0 0.0.0.255
!

! The translation of INSIDE Local IP (ACL1) 

! to the INSIDE Global IP (pool ISP_POOL)

!

ip nat inside source list 1 pool ISP_POOL

!

Verification:

Pic. 4 - NAT from 192.168.1.0/24 and 192.168.2.0/24 Subnets.

This type of translation is not used as often as the last one. In case you wanted to use it and have more than two hosts sending traffic towards the Internet, you would use the same configuration including the 'overload' keyword like presented in the scenario 3.

Scenario 3 - Dynamic NAT using IP Address Pool with Overload.
The same method like presented in scenario 2 but used when there is no sufficient public (Inside Global) addresses for the number of hosts used in our LAN (Inside Local addresses).

R1 NAT Configuration:

!

interface Loopback1
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
!

interface Loopback2
 ip address 192.168.2.1 255.255.255.0
 ip nat inside

!

interface FastEthernet1/0
 ip address 172.16.1.1 255.255.255.252
 ip nat outside
!

!

! The below command defines public IP addresses in the pool.

!

ip nat pool ISP_POOL 172.20.1.1 172.20.1.2 prefix-length 30
!

!

! The ACL1 matches on both subnets, the candidates for translation

! IP addresses that match the ACL1's statements, will be NATed. 

!

access-list 1 permit 192.168.1.0 0.0.0.255
access-list 1 permit 192.168.2.0 0.0.0.255
!

! The translation of INSIDE Local IP (ACL1) 

! to the INSIDE Global IP (pool ISP_POOL)

!

ip nat inside source list 1 pool ISP_POOL overload

!

Scenario 4 - NAT Overload
This is by far the most often used translation. This method is used on all broadband connections. In this method we only need a single public IP address (the one we use on the router's interface facing the ISP).

Our Inside Local addresses are: 192.168.1.0/24 and 192.168.2.0/24, and the single Inside Global address is: 172.16.1.1. Check the configuration below:

R1 NAT Configuration:

!

interface Loopback1
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
!

interface Loopback2
 ip address 192.168.2.1 255.255.255.0
 ip nat inside

!

interface FastEthernet1/0
 ip address 172.16.1.1 255.255.255.252
 ip nat outside

!

! The ACL1 matches on both subnets, the candidates for translation

! IP addresses that match the ACL1's statements, will be NATed. 

!

access-list 1 permit 192.168.1.0 0.0.0.255
access-list 1 permit 192.168.2.0 0.0.0.255
!

ip nat inside source list 1 interface FastEthernet1/0 overload

!

Verification:

Pic. 5 - NAT Overload.

In case things do not work, use the following steps to troubleshoot NAT:

1. Check the 'ip nat inside' and 'ip nat inside' statements on the interfaces.
2. Check if your ACL matches on the appropriate Inside Local addresses. If you send traffic from these, 'show access-list' should show you the hits against the ACL entries.
3. You could consider using extremeley dangerous command: 'debug ip nat'. This one however, will inevitably crash your production router. If the traffic is not heavy (no users using the Internet), you can try to use it the way I present below.

I will use the debug ip nat for the presentation purposes only. DO NOT USE THIS on the production equipment!

I will trace a specific Inside Local address (192.168.1.1) translation.

Step 1
Configure the ACL that matches on our single Inside Local address (192.168.1.1)

R1 Configuration: 

!

access-list 99 permit host 192.168.1.1

!

Step 2
Using ACL 99 configured in step 1, I will use debug to see the NAT translation work. The proper output is shown below:

Pic. 6 - NAT Troubleshooting.

Note!
Translation: 192.168.1.1->172.16.1.1 when the packet is sent out F1/0. The translation on the packet returning: 172.16.1.1->192.168.1.1.


Also, the ACL receives the hits. Loot at this below:

Pic. 7 - ACL Hits.

Note!
My ACL 1 which matches on Inside Local addresses has received a hit. It is configured correctly for NAT.

Lesson 53 - Network Address Translation Part 3

In my last post I tried to explain the main principles behind NAT. As an example I used static and dynamic approach in which a client receives multiple IP addresses from the ISP (Internet Service Provider). In most cases though, we receive only a single IP address which is used on the router's interface that faces the Internet.

In that case, how a single IP address can represent (be used by) multiple computers in order to allow them communication with the hosst on the Internet? The solution is to use NAT Overload.

NAT Overload
In this method, the major points described in previous lesson do not change. Still, the router will have configuration that allows is to distinguish the 'nat inside' and 'nat outside' interfaces.

Also, the router is going to replace an 'inside local' IP address with the 'inside global' (the one used on the interface connected to ISP). This time though, ALL inside local addresses (private ones; RFC 1918) will be represented by the same unique public IP address (e.g. 86.46.1.10). This will instigate the problem when the packets are sent back from the Internet. They all point to the SAME public IP. That is why, we need something else to distinguish between PC1 (e.g. 192.168.1.1) and PC2 (e.g. 192.168.1.2) that will be represented by the same address: 86.46.1.10.

What could be this distinguisher?

TCP/UDP ports of course! Recall, that each computer, while sending something out, picks a source port from the range above 1023, and the well-known destination port. These are enough to distinguish PC1 from PC2, since it is very unlikely they will pick the same source port. If that happens, the router will replace this source port in TCP or UDP header with something unique.

Here's what happens when PC1 sends packets towards www.ciscco.com.

Step 1

The packet from PC1 (src: 192.168.1.1) arrives at the 'nat inside' interface (f0/0). The router is instructed to read its source IP address as well as its source TCP port in the layer 4 header. Appropriate entry is created in the NAT table as per Pic. 1 (Inside Local). Note, that this time R1 makes a note of the source port: 192.168.1.1:2001

The IP source address is removed and replaced with the 86.46.1.10 (R1's 'nat outside' interface - S1/0). The entry in the NAT table is created under the 'Inside Global' column. Also, the 'Outside Local' and 'Outside Global' entries are populated as per Pic. 1. This record in the NAT database, will allow to translate the packet back when www.cisco.com is replying to the sender (PC1).

Notice!

The :2001 is the port number in the TCP header.

Pic. 1 - NAT Overload - PC1's Transmission.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Step 2
When www.cisco.com (72.163.4.161) replies, the packet is delivered back to R1. The router, upon receiving the packet on the 'nat outside' interface, must now find the appropriate record in the NAT table, allowing it to locate the original sender's IP address and its source port (here: 192.168.1.1:2001). If there is no such entry, the packet is going to be dropped. Since in our case, the entry exists, the router is translating the packet back as shown in the pic. 2:

Pic. 2 - NAT Overload - PC1 Receives the Reply from www.cisco.com.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Suppose during this transmission, (PC1-to-www.cisco.com), PC2 is also going to send something towards the Internet. In order to simplify our discussion, PC2 is going to send the packet towards www.cisco.com as well (but this could be any host on the Internet).

Step 3
R1 receives another packet on its 'nat inside' interface (f0/0). It is sourced by IP address 192.168.1.2, the source TCP port 1408 this time. This port has not been used yet (does not exist in NAT table under 'inside local' column), so it is unique and can be used to send the packet out. Another entry is going to be created as per pic 3 below. Watch carefully the second record in the NAT table.

Pic. 3 - NAT Overload - PC1's Transmission.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Step 4
The web server (www.cisco.com) is sending the reply back. Again, R1 will look for the appropriate entry in the NAT table ('inside global=86.46.1.10:1408), to find out which machine was the originator of the traffic flow. Translation back to the original values is performed (dst: 192.168.1.2:1408). PC2 receives the reply from the web server.

Pic. 2 - NAT Overload - PC2 Receives the Reply from www.cisco.com.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

In case the source port has already been used by some computer (even though it is rare it can happen), the router is going to use a different port. Imagine that the PC2 has also chosen to use the port 2001. In such situation, the 'Inside Local' entry is going to look like this:
192.168.1.2:2001

But, then upon noticing the port is already in use, the router will pick some other port that has not been used yet. The corresponding entry in the 'Inside Global' will be be changed to this for instance:
86.46.1.10:2002

In such case, the cisco web server will be replying to two different destinations:
86.46.1.10:2001 translated back to: 192.168.1.1:2001
and
86.46.1.10:2002 translated back to:192.168.1.2:2001

I hope this will do for you. In my next post, I'm going to show you how to configure NAT using four different methods:

• Static NAT (one-to-one)
• Dynamic NAT using pool of addresses
• Dynamic NAT overload
• Dynamic NAT using pool of addresses with overload

Lesson 51 - Network Address Translation Part 1

Why NAT?
In 90s we observed an exponential growth of users connecting to a global network called the Internet. As of today, there are billions of people in this global village. The problem is that the designers of TCP/IP protocol suite did not realize this would happen. With the 32 bit IP address that was used originally we could allocate more than 4 billions unique identifiers. Apparently, this is not enough today having billions of devices using public network.

In February 1996 RFC 1918 document has been published. In it, some IP address reservations have been made known as PRIVATE ADDRESS SPACE. The ranges have been reserved as follows:

• 10.0.0.0 - 10.255.255.255  (10/8 prefix)
• 172.16.0.0 - 172.31.255.255  (172.16/12 prefix)
• 192.168.0.0 - 192.168.255.255 (192.168/16 prefix)

Private Address Space is used in our local networks (home, work etc.) and cannot be used as public IP address range. This scope is sometimes referred to as non-routable addresses. This does NOT mean the routers cannot use them to route the packets. It means, that these addresses should never appear as source or destination on the Internet (unless this is some form of attack or misconfiguration).

What is NAT?
Network Address Translation is a technique in which the EDGE router (the one that is connected to ISP (Internet Service Provider) and your LAN, removes the original SOURCE address in the IP header (private range - RFC 1918) and replaces with legitimate, unique, public one leased to us by ISP while sending packets towards the Internet. However it is smart enough to keep this information in the special database called: NAT table. When the reply comes back, the SOURCE and DESTINATION addresses are reversed in the IP header. Once the packet reaches our EDGE router, it uses the right entry in the NAT table to swap the destination IP PUBLIC address (that represented our computer) back to its PRIVATE address.

If it sounds a little vague right now, do not worry since in my next post I will show you this operation step by step using some graphics.

NAT Terminology
If you want to fully understand this technique, you must understand the terminology that is used by it first. The official terms can be found on Cisco web site:
http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080094837.shtml

Look at the below picture I'm going to use in the NAT section of this tutorial.

Pic. 1 - NAT Topology

Icons designed by: Andrzej Szoblik - http://www.newo.pl

What you are looking at is two companies (ABC and XYZ) connected to the Internet represented byR3 here. In real life this cloud consists of many routers belonging to different ISPs.

Computers are connected to the LAN and in both cases (I did this on purpose), use the same range of Private Address (192.168.1.0/24). Of course they can use any of addresses listed in RFC 1918.

R1's Serial0/1 interface uses 1.1.13.1, and R2 uses 1.1.12.2 address. Both are NOT described as the PRIVATE so they are PUBLIC or legitimate ones (my apologies if I used somebody's real, public addresses here. They are only used for educational purposes).

Using NAT technique you must be able to properly name them as per Cisco article above. Let's assume that we work for ABC company and we look at all addresses from this standpoint (this is critical). Here's what they are as per Pic. 1:

ABC Company

• INSIDE LOCAL - address (inside your LAN, before translation). In our example this would be our 192.168.1.0 (ABC) address.
• INSIDE GLOBAL - address used on the router's interface facing the Internet. GLOBAL means that is is legitimate, globally unique address. These are provided by ISP or Network Information Center (RIPE in Europe, ARIN in the US, etc.). For instance, in the example this would be 1.1.13.1 address.
• OUTSIDE GLOBAL - address which is also globally unique (like inside global), but is leased to another company, not us (remember? We're ABC here). In this example this would be 1.1.12.2 for instance.
• OUTSIDE LOCAL - address of the outside host as it appears to our local hosts. Typically in the NAT table we see them as OUTSIDE GLOBAL addresses. Our R1 router will never see XYZ company's 192.168.1.0 scope since it will be translated into a public IP before they send packets towards the Internet.

If you want to read more in-depth description of the terminology please, read the Cisco article I included in this post.

This concludes the introduction to NAT. In my next post, we will inspect step-by-step the process of NAT on the router.