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First we look at what exactly NAT is? It means Network Address Translation. This technology or technique was developed to overcome the shortage of public IP addresses in IPv4 standard. With the invent of NAT, the problem of shortage of public IP addresses has been solved quite a bit. Now we have moved to IPv6 which has a lot more addresses than IPv4 but still most of the world uses IPv4 in combination with NAT.

If anyone wants to dig down deep to understand NAT, he can go to the following Wikipedia article:

http://en.wikipedia.org/wiki/Network_address_translation

Now let’s start by configuring a basic NAT on a Cisco router. The type of NAT we are going to configure here is Overloading. This means that we will map one public IP address to many private or local IP addresses.

 

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As the title suggests, I want to have a limited access to my router through telnet. I want only specific networks to be able to telnet my router. All others should be blocked from telnetting the router.

So let’s start:

Enter global config mode

Certhacker#configure terminal
Enter configuration lines, one per line. End with CNTL/Z
Certhacker(config)#

Create Access list to permit a network

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Hostname is the name of the router. It’s up to us what we name it. It’s like giving a name to our own systems. If we are using a command line interface to access the router or switch, then we can see the current name of the device before the prompt.

 

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SSH is the preferred protocol for remote communication with Cisco products. SSH is more secure than Telnet because all the data is sent with encryption.

To see if SSH in Cisco router or switch is enabled or not issue the following command:

Certhacker#show ip ssh

%SSH has not been enabled

You can see that SSH has not been enabled on the router or switch. To enable SSH do the following:

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Understanding How InterVLAN Routing Works

Network devices in different VLANs cannot communicate with one another without a router to route traffic between the VLANs. In most network environments, VLANs are associated with individual networks or subnetworks.

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Cause Analysis

Routing table of Corp Router showed RIP entry instead of EIGRP.

Network Diagram

Handaling Process &  Solution

For resolving this problem I undertook the following steps:

 

1)    After applying redistribution on R3, I checked that the router 871W and hostd were able to see all the routes on the other side of R3 as RIP routes.     That was normal.

2)    Then I checked Corp Router and found out that Corp showed the route 10.1.12.0 as a RIP Entry. Corp Router was basically running both RIP and EIGRP. Static routes were also installed in Corp with AD of 150.

3)     I stopped RIP on Corp, and checked again. This time a static route took the place of RIP Entry for 10.1.12.0 network.

      4)   I checked the topology table of EIGRP and found the following error:

 

 

The route to 10.1.12.0 showed “FD is Inaccessible” which meant that the EIGRP route in the topology table for the network 10.1.12.0 was not being used for the routing table.

      5) After that I removed static entry as well. EIGRP route was successfully added to the routing table with AD of 170 as shown below

 

         

     6) The route from redistribution is considered as an external route in EIGRP. That’s why we saw RIP and static route in the routing table instead of   

          EIGRP.