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Saturday, March 23, 2013

How to Set The IP Address on a Cisco Router Interface

How to Set The IP Address on a Cisco Router Interface

This article will discuss how to choose which interface needs to be configured on a Cisco Router, as well as how to go about configuring that interface

Setting the IP address on a Cisco Router interface is necessary for the router to be able to communicate with other networks, as well as its own network.

Prior to setting the IP address, the administrator needs to be aware of the address that will be allocated to the router. In many cases this will be dictated by a subnet mask that is already in use, or may be pre defined by either the network manager or service provider.

Secondly, the administrator needs to be aware of which interface needs to be configured. Most Routers only have a couple of interfaces and this is easy to determine, however some larger enterprise routers, such as those found in an ISP may have many interfaces. Configuring the wrong interface with the wrong IP address is a sure fire way to bring down your network and cause an outage.

Interfaces on modern modular routers are divided into slots, and these in turn are separated into interfaces. These are numbered on the back of a router to be physically inspected, and can also be listed from the command line with a show ip interface brief command

We can see that Interface FastEthernet 0/1 already has an IP address and that interface FastEthernet 0/0 is currently un-configured

Likewise, you would expect to have access to an up to date network diagram which should show you the ports and their connection partners. A show cdp neighbors command can also be used to determine who the routers existing neighbors are:

We can see from the above that we already have a connection to router R3 going out interface Fast Ethernet 0/1 . This confirms the information learned above with the show ip interface brief. 

It can also be a good idea to ping the IP address to be configured to ensure that its not in use.

Thus, It can now be determined with some certainty that interface FA0/0 is the interface to be configured

Once you have determined your interface to be configured, the actual configuration process is very easy. the syntax is as follows:
  1. enable to enter into privileged mode. (enter any passwords required)
  2. config terminal to enter into global configuration mode
  3. interface (interface) to enter the configuration mode for the desired interface
  4. no shutdown to bring the interface online
  5. ip address (ip address) (Subnet Mask)  to assign the desired IP to the interface.

For a serial interface, especially in a lab environment you will need to set the clock rate for the port. this is done using the clock rate (clock rate)  command on the interface.

And that's all there is to it! for an inside facing interface, this is all that is required to bring the interface online and you should now have connection to the network. this can be tested with a ping command.

For an outside (external) interface, assuming that all is set up on the other end OK then connectivity will also have been established

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Thank you for reading and please check back in again soon for more articles.

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