IP Overlapping - Who can help me understand

[techspec01]

Hi, currently doing a lab in PT. Getting the following error message:

FIRST Router(config)#interface Fastethernet0/0

Router(config-if)#ip add 172.16.0.1 255.255.240.0

Router(config-if)#exit

SECOUND Router(config)#interface Fastethernet0/0

Router(config-if)#ip add 172.16.0.2 255.255.240.0

Router(config-if)#exit

Now I'm trying to connect the SECOND router to a THRID router:

Router(config)#interface Fastethernet1/0

Router(config-if)# ip add 172.16.0.17 255.255.240.0

% 172.16.0.0 overlaps with FastEthernet0/0

Since the CIDER is /30 isn't the magic number 16?   

172.16.0.0 to 172.16.0.15

172.16.0.16 to 172.16.0.31

~

~

Wouldn't 172.16.0.17 be a valid IP address for 2nd Router Fa1/0?

Can you please help me understand what I'm missing? thank you

[Richard Burts]

I am not clear about this statement

Since the CIDER is /30 isn't the magic number 16?

How did you believe that the CIDER is /30? The mask for /30 is 252. And for /30 the magic number is not 16.

Using 172.16.0.17 would work if the mask you were using was 255.255.255.240 but you are using mask 255.255.240.0. With that mask you have addresses beginning at 172.16.0.1 through 172.16.15.255 

[omz · Accepted solution]

CIDR for 255.255.240.0 would be /20

172.16.0.1 255.255.240.0 gives you 172.16.0.1 - 172.16.15.254

172.16.0.1 255.255.255.240 gives you 172.16.0.0 - 172.16.0.15

the MAGIC number you are referring to would map to the octet .. 

255.255.255.240 = / 28

172.16.0.0 - 172.16.0.15 

172.16.0.16 - 172.16.0.31

172.16.0.32 - 172.16.0.47

255.255.240.0 = /20

172.16.0.0 - 172.16.15.254

172.16.16.0 - 172.16.31.254

172.16.32.0 - 172.16.47.254