Wednesday, January 25, 2006

 

Sharing IP addresses on interfaces. (Question #1)

Let's start things off with a simple one.

Note how Cisco IOS will prevent you from configuring the same IP address on more than one interface. For example


Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#int ethernet 0
Router(config-if)#ip address 10.10.10.1 255.255.255.0
Router(config-if)#no shut
Router(config-if)#int serial 0
Router(config-if)#ip address 10.10.10.1 255.255.255.0
% 10.10.10.0 overlaps with Ethernet0
Router(config-if)#


Now, given this fact can you tell me what legal configuration results in the following output that clearly shows the same IP address on two different interfaces?


Router# show ip interface brief

Interface IP-Address OK? Method Status Protocol
Ethernet0 10.10.10.1 YES manual up up
Serial0 10.10.10.1 YES manual up up


There is enough information here to answer the question. If you need any more information or if you think you know the answer, use the comments for this blog entry to communicate that.


Comments:
unnumberd to Ethernet0
 
Yes, that was it.
 
hi mr da gama ı am newby at cisco/ccna
ı have question why am ı use 2 ip adress (same) different interface...thanks..sorry my englis
 
You use it to save ip address space in case you have a point to point link
 
Wht does anonymous mean by unnumberd on Ethernet0?
 
int s0
ip unnumbered e0
 
I am also a newbie so I apologize if this is a simple question but I have tried as many places as I know to find the answer. The question is:

1. I am trying to set up a network using Router SIM. I have configured my s0/0/0 line using the command below. Worked fine.

2. When I tried to configure my other interface f0/0 I get the "overlaps with serial 0/0/0" message below. I also get this on subsequent configurations.

3. I know this is a private address space, the mask required does not make sense to me. I have tried incrementing the ip address but that does not work either.


RTA(config)#int s0/0/0
RTA(config-if)#ip address 192.168.2.1 255.255.0.0
RTA(config-if)#no shut
13:53:39 %LINK-3-UPDOWN: Interface Serial0/0/0, changed state to up
13:53:39 %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0/0, changed state to up

RTA(config-if)#int f0/0
RTA(config-if)#ip address 192.168.1.1 255.255.0.0
% 192.168.0.0 overlaps with Serial0/0/0
RTA(config-if)#ip address 192.168.1.0 255.255.0.0
% 192.168.0.0 overlaps with Serial0/0/0
RTA(config-if)#
RTA#sh ip int bri
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES unset administratively down up
FastEthernet0/1 unassigned YES unset administratively down up
Serial0/0/0 192.168.2.1 YES manual down down
Serial0/0/1 unassigned YES unset administratively down down
Serial0/1/0 unassigned YES unset administratively down down
Serial0/1/1 unassigned YES unset administratively down down


Can anyone help??
 
This is in response to anonymous, who is getting the overlaps... error.

Your interfaces are on the same network. Change the subnet mask to 255.255.255.0 and that will fix it.

Remember, each interface must be on it's own network normally, except as show in this post, where addresses are being saved. Don't let this post confuse you. Use unique networks on each interface.
 
when you use ip unnumbered command on the serial interface you obtain this result.
 
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