connecting 2 routers

I'm reading the Cisco press book by Odom (INTRO) and it's really confusing me. I found a similar image here, I hope I can explain it.

How do you assign ip addresses to two inerfaces used to connect two routers?

[/img]http://www.techexams.net/images/accesslist2.gif (alt+p) -Where did we get 172.16.11.253? -Why are we not using a 192.168 addresss for S1? -Which address do we use for S0 and why?

(alt+p) -Where did we get 172.16.11.253? -Why are we not using a 192.168 addresss for S1? -Which address do we use for S0 and why?

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Comments

binarysoul

sorry folks, here is the link to the image:

http://www.techexams.net/images/accesslist2.gif

DragonNOA1

binarysoul wrote:

-Where did we get 172.16.11.253?
-Why are we not using a 192.168 addresss for S1?
-Which address do we use for S0 and why?

172.16.x.x is another private address. B/c routers seperate different subnets, then each interface should be different numbers/subnets. The 172.16.x.x number is to make sure you see the change (192.168.1.0 and 192.168.11.0 might be hard to notice) and to help you realize that the change can be to any number.

The "S0" number depends on the addressing scheme but it could easily be 172.16.11.1.

binarysoul

B/c routers seperate different subnets, then each interface should be different numbers/subnets.

This is very interesting.

agustinchernitsky

S0 and S1 on router X and Y should be on the same subnet (you should use a subnet like /30 to use only 2 hosts and save address space).

If they are not, you can't route (they won't see each other). In the example they use private addresses to show you they are different subnets (A) from the other interfaces.

Hope it helps

Danman32

Assuming you are trying to conserve addresses, S0 on router X is probably going to be 172.16.11.254. Clearly you can't use 255, that is most likely the broadcast address for the subnet, since the octet is all ones. One less than that is 254, which we haven't used yet, and right behind that is 253 which the other router is using. 252 is most likely the subnet network address assuming a subnet /30.

If you weren't given any numbers, you could put in whatever floats your boat. But often, especially in test questions, you want to conserve IP addresses and not waste them, so point-point links with only 2 hosts will use a /30 subnet.

BubbaJ

I'm surprised that Cisco has not started using /31 masks for the CCNA exam yet. For years, they have had this for point-to-point connections, and it can be found in the CCIE exams. I wouldn't really consider it CCIE-only level material.