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How to troubleshoot NAT / PAT
Network address translation (NAT) is a function by which IP addresses within a packet are replaced with different IP addresses. PAT translates from one network address to another network address and includes a port number so that one real address can handle up to 4,000 private addresses. This article describes the solution of some common problems in NAT/PAT. A network administrator can troubleshoot these problems like connection problem in NAT and translation of packets in translation table. |
Common problems and their solution in NAT/PAT
Basic verification steps of the NAT operation.
Solution. You follow these steps.
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You verify that the correct translations exist in the translation table in NAT.
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Clearly define what NAT is supposed to achieve.
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You use the “show and debug” commands to verify the translation is occurring or not.
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You review in detail what is happening to the packet in the translation table?
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You verify that routers have the correct routing information to move the packet along.
Problem # 1. If the translation not installed in the translation table
Solution.
If you find that the appropriate translation does not get installed in the translation table.
- You have to done the configuration correctly.
- The access list referenced by the NAT command allows all necessary networks.
- Your Nat pool having enough addresses.
- Make sure that the router interfaces are appropriately defined as NAT inside or NAT outside.
Problem # 2. If the translation entry is not correctly used in NAT table.
Solution. You verify the following steps if translation installed in the table correctly but is not used.
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You should verify that there are not any inbound access lists that deny the packets from entering the NAT router.
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You verify there is a route to the destination as this is checked before translation for packets going from inside to outside
Problem # 3 If NAT is operating correctly but still having connectivity problem.
Solution.
When you found that NAT operating correctly but still having connectivity problems.
You verify the following steps.
- You verify that the layer 2 connectivity is ok.
- After this step you verify that the layer 3 routing information is ok.
- You must search for packet filters that could be causing the problem.
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Problem # 4. The NAT computer is not properly translating packets.
Solution.
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The first step is that you verify the interface on the Windows Router that connects to the Internet is added to the Network Address Translation (NAT) routing protocol.
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You verify that the Public interface connected to the Internet option on the General tab on the properties page of the Internet interface is selected.
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Verify that the Private interface connected to private network option on the General tab on the properties page of the private network interface is selected.
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If you only have a single public IP address, you verify that the translate TCP/UDP headers option on the General tab on the properties page of the Internet interface is selected.
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When you have multiple public IP addresses, you verify that they are typed correctly in text boxes provided on the Address Pool tab on the properties page of the Internet interface.
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You verify that the IP packet filtering on the private network
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You verify that TCP/IP filtering on the private network and Internet interfaces is not preventing the receiving of traffic.
When you using NAT for VPN.
Solution.
If your connection uses NAT or PAT.
You must check the “Use NAT Transparency” box in the “Login Properties” setting up the connection.
If your connection does not use NAT. Do not check the box.
You make sure that the “NAT Port:” is set to 80. Some versions of the client default to 80, in transparency mode, packets are sent from the client to the gateway with a destination port of 80 and a source port of 500 which tells the gateway to do things in a way that will work through a NAT or PAT device. For this to work, your NAT or PAT device must preserve this information in both directions.
This article provides the solution of some common problems in NAT/PAT protocols. A network administrator can solve these common problems with the help of above mention steps that describes in this article.
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