You are right, that when setting up an unnumbered interface, you do referance another interface on the router with an IP. But the unnumbered interface will not have an IP address directly assigned to it.
When the router sends packets out the unnumbered interface, it will use the IP address of the referenced interface as the source IP. So it's like 2 interfaces have the same IP, but not really. A router will never allow you to assign the same IP to multiple interfaces.
I don't know a lot about this option. And I'm not quite sure why you would use it. I guess you save a couple IP addresses. There are probably some other specialty situations as well.
From my limited understanding, IP UNNUMBERED is used when there are many point-to-point connections terminating on the same router, in order to save address space (especially with things like PPP connections for internet dialup, xDSL etc).
I believe its quite common to use a loopback interface with this option (as it is always up and available)
I found this Cisco doc on using IP unnumbered.
You will see that it is quite old, it was popular to conserve IP address (as has been stated in previous postings). This was before VLSM and RFC 3021 (using 31 bit prefixes on point-point links). However I recall that it was problematic if you used SNMP to manage you network (can't recall why).
Hi Charles and Haines,
Thanks for your contribution.
As far /31 issue is concerned, finally now I understand your points very well.
But this leads me to another confusion..
My ISP provides me an IP with 32-bit mask. This is dynamic IP.
ipconfig statement on my PC shows that IP is provided on WAN ( PPP/SLIP) Interface.
Now the doubt is When do we use /31 and when do we use /32 ?
Because both can be used on PPP links to preserve IP address space.
It would have to be implemented on the ISP side in this scenario for it to work. You have to have the same subnet mask on both ends of the PPP link! But you can use the /31 on all of the PPP links in your autonomous system! Does this help?
By the way Chetan! Good job on the Frame Relay post! I would have awarded you point! LoL
For the purposes of CCNA the answer is ALWAYS /30 or 255.255.255.252 for PPP links!
Anytime you would use a /30 in the real world you can instead use a /31 or .254 as long as you follow the requirements I specified in my document! In other words, it will not work if routing with RIP version 1!
Basically, you can just use it on your PPP links! ISP's use /31 PPP links to connect their MANY routers! Remember, they have a lot of them and the benefit of conserving two addresses per PPP link is a big deal for them!
So, to answer your question.. You would use a /31 if you work for an ISP, a /31 is used for all PPP links per your comany's schema, or you just feel like playing around with your home lab! Try it.. even works in Packet Tracer 5!!!
255.255.255.255 is used for the router loopback address(es). This is a interface which is up when the router is up unless somebody explicity shuts it down. Since it exists in isolation it has a mask of 255.255.255.255. The Loopback interface is used for inter-router applications such as routing, GRE etc tunnels and DLSW peers.
You are too fast..I was also writing the same reply..after doing some googling..
But you win the race..so I am editing my post
Other than Loopback address, some ISPs also use /32 bit mask when providing dynamic IP to the clients.
My ISP do this..
I have wrote this earlier also.
But I forgot, How it actually works..?