Usable subnets

Hi, when calculating "usable subnets" and you have 7 subnet bits. Is the correct answer 126 usable subnets or 128? is subnet zero and the broadcast usable? I know hosts are minus 2.
Thanks
Phil

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elegua

fillthy wrote:

Hi, when calculating "usable subnets" and you have 7 subnet bits. Is the correct answer 126 usable subnets or 128? is subnet zero and the broadcast usable? I know hosts are minus 2.
Thanks
Phil

Hi.

No, the first IP is the network address and the last one is the broadcast ip address.

Hope this help.

flares2

Phil,
When you say 7 subnet bits I assume you are talking about a Class C /31. A /31 is not a good example for this particular question, due to hosts. Remember to 2n-2 formula for calculating hosts? In this example if we have 7 subnet bits, then we have only 1 host bit. Thus in the forumla 2 to the first power minus 2 equals 0. Subnets are pretty irrelevant in you have no hosts.
So let's explain this using a /30. We have 6 subnet bits and 2 host bits. For hosts it would 2 to the second power minus 2 which equals 2. (The two that are subtracted are the network address and the broadcast, like elegua said) leaving two usable ips for hosts. Using this same formula for subnets, 2 to the 6th power equals 64, i.e. 64 usable subnets. This is assuming your device is running IP subnet-zero. If it is not, then once again you would subtract 2 (the first and last subnet) and get 62 usable subnets, or if it is not, you can enable it using the command (config)#ip subnet-zero.
There's a bunch of good sites for subnetting, I don't remember them off hand. Check the FAQ and technotes, I'm sure there's plenty of subnetting tips there.
Flares

floppydisk

i don't think IP Class matters in his question here....he simply asked how many subnet can a /25 obtained and if it should follows the 2 to the N - 2 formula....if he supply an IP address then yes we'll have the class and subnet rules in place.

fillthy

Hi, heres the actual question I was on about:

You need to assign the first available host address on the first available subnet for IP address 147.65.0.0/23 to the routers E0 interface and the next available address to the HTTP server. Which address should you assign to the HTTP server?

147.65.0.2
147.65.1.2
147.65.2.1
147.65.2.2
147.65.254.2

There was a little diagram but all it showed was a router with subnet of 147.65.0.0/23 on its E0 interface. I originally answered 147.65.0.2 since that was the first subnet (subnet zero) but I don't think you can use that subnet.
Thanks
Phil

flares2

floppydisk,
Class does matter because of the amount of available usable bits. In phil's example he stated 7 subnet bits implying a /31 (if it's Class C). A /25 would be 1 subnet bit, 7 host bits. The reason class is relevant is because, in your example, a Class C /25 (8 usable bits) there would be 2 subnets (.0 and .12

with 126 hosts per (.1-.126, .129-.254) In a Class B /25 (16 usable bits) there would be 512 subnets (.0.0, .0.128, .1.0, .1.128 etc) with 126 hosts each (.0.1-.0.126, .0.129-.0.254, .1.1-.1.126, .1.129-.1.254, etc) I agree having an IP address example would help. I was trying to give an overall explanation.
Flares

flares2

Phil,
Thanks for posting a specific example, I think it'll make this easier on all of us. The question asks to use the first available IP on the first available subnet.
Using ip subnet-zero your first available IP and subnet would be 147.65.0.1. Because this is not an option, we can assume the subnet-zero is not being used. The next available subnet would be 147.65.2.0, with its first available IP of course being .1 So the answer should be 147.65.2.1.
Sorry my last post asking for an example was after you gave the example, I starting my post then got pulled away on a trouble call, by the time I got back and posted, your post was already there.
Flares