More Subnetting Fun

Johnny Johnson

Came across this question and am stumped:



I'm thinking you want /25 for Los Angeles, /26 for Denver, /27 for Phoenix, and /29 for Seattle. The only problem is that there are multiple choices when it comes to those. An explanation on which one to choose why would be greatly appreciated!

dynamik

Start with 192.168.50.0/25, so you'll have 000000000 - 01111111 (0-127) available for use. That will give you 126 usable hosts, which meets your LA requirement of 99.

The next one will be 192.168.50.128/26, so you'll have 100000000 - 10111111 (128-191) available for use. That will give you 62 usable hosts, which meets your Denver requirement of 43 hosts.

Then, you'll be using 192.168.50.192/27, so you'll have 11000000 - 11011111 (192-223) available for use. That will give you 30 usable hosts, which meets your Phoenix requirement of 17 hosts.

Finally, you'll be using 192.168.50.224/28, so you'll have 11100000 - 11101111 (224-239) available for use. That will give you 14 usable hosts, which meets your Seattle requirement of 7 hosts.

You can't use /29 because that only gives you three bits to use, which gives you 8, but you have to subtract 2 because they're reserved for the network and broadcast addresses, so you'll only end up with 6 usable.

It's a bit early for all of this, so hopefully that's right (or at least close).

dynamik

Also, be there or be square: http://www.techexams.net/forums/mcsa-mcse-windows-2003-general/43776-anyone-interested-free-virtual-training-me-2.html#post320017

Johnny Johnson

I always get confused on whether or not to subtract 2 when the question says that CIDR is being used and the use of subnets that contain all 0s and 1s. What does that mean exactly?

dynamik

You ALWAYS subtract two from the host range to get the usable IP addresses. Those denote the network and broadcasts addresses. You only subtract two from the number of subnets if you aren't allowed to use the 1s and 0s subnets (i.e. no ip subnet-zero in Cisco parlance).

For example, suppose you have 192.168.5.0/26 to work with. You'll have these for subnets:
00000000
01000000
10000000
11000000

The first and last are what they're referring to with the all 1s and all 0s subnets.

Tyrant1919

Where'd this question come from. MSPress? the format looks familiar.

What's really cool, is when you see a subnet mask of 255.255.255.252 in use on a computer. That's when you know you're in deep subnetting @#$!.

dynamik

It's from Transcender. I'm not sure how they feel about having screenshots of their questions post on forums. As long as it's just one or two, I suppose you can consider it advertising

Johnny Johnson

LOL! Yep, it's from Transcender. Dynamik has obviously seen that question several times.