subnetting question

hcclnoodles

does sombody have an easy to remmber formula to resolve the questions that as how many subnets and hosts are available from this specified network address/mask

eg ...

Q: You have subnetted a network address (213.105.72.0) with a /28 mask.. How many usable subnetworks and usable hosts (on each subnetwork are avaialble)

A: 14 networks and 14 hosts per net


I just need an easy way to work these ones out

cheers
Gary

Jerz

Well... I'll show you the way I like to figure these out:

/28
255.255.255.11110000
host = 4bits = 2^4 = 16-2 = 14 hosts
subnet=4bits = 2^4 = 16 (-2=14 depending on the question)

hcclnoodles

sorry why is depending on the question? im slightly confused why you have deducted 2 from the subnets as well as the hosts

sorry to be a pain but could youu explain further

Jerz

Have you even cracked a book?

Do yourself a favor and watch the videos at this site:

http://www.learntosubnet.com/

nethead

The -2 from the subnet relates to whether or not you allow subnet-zero. This is an old issue nowadays and generally all subnets are used.

However I'm not sure what angle the CCNA takes on this now, you'd need to read a book for that.

hcclnoodles

i have "cracked a book", but i cant undestand why the you lose two subnets

if you can subnet a /28 into 16 subnets, of 16ips each i can understand the loss of the netwoek and broadcast addresses from each subnet, but i cant understand which of the 16 subnets become unusable? Im pesuming the first and last one become unusable, my question is why ? why does a subnet containing ip addresses 0-15 in this case become completely redundant....say if you were to subnet to /25 with only 2 subnets available would you consider the first subnet of 0-127 as unusable ??

hcclnoodles

thanks nethead, that explains it a bit further, it seems all the ccna study material i am reading considers the first usable subnet to be the seconf actual subnet, I would really like to understand the reasoning behind this... secondly, you say the zero subnet is unusable, thats 1 subnet, where does the second one come from? (is it the last one, if so again...why)


cheers

Webmaster

The reason for subtracting the two is that otherwise the network address of the first subnet (ie. 192.168.1.0) is also the network address for the entire 'unsubnetted' network, and the broadcast address for the last subnet (192.168.1.255) is also the broadcast address for the entire 'unsubnetted' network. So if there is one device that doesn't support subnet-zero and all 1's addresses, you'll get conflicts when not subtracting the two unusable 2 subnets. I.e. this device will use the 192.168.1.0 to refer to all the addresses in this Class C network while if subnetted it should refer to only the addresses in the first subnet.

hcclnoodles

Ok I understand, but if that is the case, what happens when you have a /25 mask, ie 2 subnets in the 4th octet, each of 128 ips. Does this mean the first subnet is unusable and the last is unasble ?? ......but that kind of leaves no subnets at all

NetMan says they are not used anymore, why is this, was the risk that Webmaster told us about considered to be too small compared to the potential wastage of IP's and the effective redundancy of some subnet masks (ie /25 above) are are there other reasons...I would really love to find this out

Also, most importantly, the CCNA certification study guides sort of suggest to me that the -2 is for subnets also (hence me raising this question in the first place) but exactly how is it implemented, I again refer to my /25 example above and does it only come into play after certian conditions are met

Nay help on this would be greatly appreciated

Webmaster

Does this mean the first subnet is unusable and the last is unasble ?? ......but that kind of leaves no subnets at all

Exactly, hence /25 couldn't be used.

NetMan says they are not used anymore, why is this, was the risk that Webmaster told us about considered to be too small compared to the potential wastage of IP's and the effective redundancy of some subnet masks (ie /25 above) are are there other reasons...I would really love to find this out

The conflict was bound to occur. Perhaps the following quote from an older post will clear it up:

Webmaster wrote:

In traditional subnetting you have to substract the 2. (this is the way it is still described in most books, tutorials and online references, especially those related to certification) If your network equipment supports it and an appropriate routing protocol is being used (e.g. OSPF, RIPv2, EIGRP) you do not have to substract the 2, allowing you to assign the Subnet Zero and the All-Ones Subnet.

As mentioned in RFC1878 :
"""For the sake of completeness within this memo, tables 2-1 and 2-2
illistrate some options for subnet/host partions within selected
block sizes using calculations which exclude all-zeros and all-ones
subnets [2]. Many vendors only support subnetting based upon this
premise. This practice is obsolete! Modern software will be able to
utilize all definable networks."""

RFC 950 forbade the use of the subnetted network IDs where the bits being used for subnetting are set to all 0's (the all-zeros subnet) and all 1's (the all-ones subnet). The all-zeros subnet caused problems for early routing protocols and the all-ones subnet conflicts with a special broadcast address called the all-subnets directed broadcast address.

However, RFC 1812 now permits the use of the all-zeros and all-ones subnets in a CIDR-compliant environment. CIDR-compliant environments use modern routing protocols that do not have a problem with the all-zeros subnet and the all-subnets directed broadcast is no longer relevant.

The all-zeros and all-ones subnets may cause problems for hosts or routers operating in a classful mode. Before you use the all-zeros and all-ones subnets, verify that they are supported by devices involved.

hcclnoodles

thanks, so it seems that the further away we move from classfull protocols and technologies and the nearer toward a fully VLSM/CIDR world, the more likely it is that this issue will finally disappear..


Thanks very much for your help.. I am surprised however that with cisco's main focus on classless protocols in the later variations of the exam and their move away from the likes of RIP v1 and IGRP, that they still seem to be fully supporting the -2 on subnets in their certifications even though most new routing protocols can deal with it quite happily

I wonder if Cisco have released a stance on this issue to aid people like me who are unsure how to answer their questions relating to the "first usable subnet"

cheers
Gary