NAT Confusion

[rbsims]

I recenty took the CCNA exam and came across the following issue


You are given an ip range of 172.16.1.50 – 172.16.1.59 from an ISP. 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 (172.16.1.50 255.255.240) or must you use the classful mask (255.255.255.0)

Or can this question be answered given the information supplied??

I've tried to find an answer everywhere

Thanks in advance

 

[perryk]

I wonder whether that is a correct range. It has 10 addresses?!?! A mask of /28 (255.255.255.240) will give you 16 addresses (14 usable - exclude the network address and the broadcast address). A mask of /29 (255.255.255.248) will give 8 addresses with 6 usable.
I don't know what to make of the range in your example. Could you be more specific?

 

[burtsbees]

You can specify that range with the /28, if that's what you're asking. The range would start at .49, not .50, but specifying 172.16.1.50-172.16.1.59 netmask 255.255.255.240 would work, since those 10 addresses fall within that range. The whole range is .48=wire, .49-.62=useable IP's, .63=broadcast.

Burt

 

[perryk]

Agreed, but imo if he is going to use only some of these addresses, it's a good idea to explicitly specify the non-standard range wherever is warranted (firewalls, proxies, network shares etc etc). Otherwise it would make spoofing easier - an intruder/hacker could just assume one of the never-used addresses that fall in the mask's range.

 

[rbsims]

I just wanted to thank everyone for their input. I don't understand why Cisco would porvide a range without a prefix. In any event, if the question indicates that the ISP provided the range, it has to have a VLSM mask (unless you purchase a class c).

But thanks again, you guys are great

 

[burtsbees]

perry---only the range specified will be natted, not the other 3 addresses...

Burt