CCNA Nat question

rbsims · ‎03-25-2008

On the CCNA test I given an ip range of 172.16.1.50 - 172.16.1.59 from an ISP the netmask information was not supplied). Can you determine the prefix by adding up the number of IP addresses, and finding what VLSM block size they fit into (172.16.1.50/28) for purposes of creating an IP Nat pool (172.16.1.50 0.0.0.15) or must you use the classful mask (255.255.255.0) I played with this for a long period of time and concluded that you can only use VLSM is if both in the 4TH octet are divisible by 4. How is it possible to configure a IP Nat pool based only on the address range??

I need help

thanks

rais · ‎03-25-2008

Number of IPs would give an idea but not the answer. Once you have .50, you should think about 48 b/c it has zeros in it. You can also do the binary of 50 and figure out how much to subtract to get zeros in the binary. Then subtract the same number from 50 to get the answer [keeping in mind # of IPs in the range].

For Cisco 'ip nat pool' command you have to give 1st ip, last ip and netmask or prefix-length.

Thanks.