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New Member

subnetting

Hi all, just looking subnets, would you ever say subnet say a 172.16.0.0 class B network to say a /11 or would you always subnet above its classful mask i.e /17 ?

15 REPLIES

Re: subnetting

Hi Carl,

Now we donot have any classful addressing and every address is classless.

The above one is a Class B address and abt the network part is all how we interpret

a.It can be interpreted as a Class B address with a class A network 172.0.0.0/8 and subnetted into 172.16.0.0/12

b.It can be interpreted as a Class B address with a Class B network 172.16.0.0/16 and subnetted into 172.16.0.0/17 ..

Interpretation depends on the mask what we give after the prefix.

Pls let me know if it helps

regards

vanesh k

Blue

Re: subnetting

if you needed to 'superNet' your 172.16.0.0/16, then yes, you could do so, even to the point of /11.

one key point to keep in mind...

when you add bits to the subnet mask, of a classful boundry, that is known as subnetting.

(this has the affect of providing more subnets at the cost of number of hosts per subnet)

when you take away bits from the subnet mask, of a classful boundry, that is known as superNetting.

(this has the affect of providing more hosts per subnet at the cost of being able to define less subnets)

NOTE: superNetting also has the benefit of reducing the number of entries in a routing table. superNetting is aka CIDR (classless interDomain routing)

New Member

Re: subnetting

Hi,

The mentioned class B subnet can be called as /11 after summarization/aggregation.

For e.g. if you see 172.16.0.0/11 in a routing table, it means that all further subnets under this address range are with the Router who advertised this Summarized Route.

BGP and other protocols summarizes routes to reduce the routing tables.

HTH [Rate Plz]

Re: subnetting

Hi

Pls note that you cannot have 172.16.0.0/11 ,it can be only 172.16.0.0/12 which is one of the subnet from 172.0.0.0/8

regards

vanesh k

New Member

Re: subnetting

We can very well have supet as 172.16.0.0/11.

This might be more used in Summurization Routing/we have requirement of Very BIG network where no host are more. But that a be a remote chance we do configure BIG LAN Network as it end with highbroadcast and LAN may be useless.

For Service Providers, they used to get Supernet IP Network block in this way.

Re: subnetting

Hi ,

Pls note that you cannot have 172.16.0.0/11

Reason : considering a /8 network getting subnetted , 2^16-11 equals 2^5 and its 32 , so the value can be in multiples of 32 only

so the subnet for /11 is 172.0.0.0/11 , 172.32.0.0/11 , 172.64.0.0/11 ...

so we can have only 172.16.0.0/12

regards

vanesh k

New Member

Re: subnetting

for the purpose of the cisco exams, in the ccna, they would not ask this I gather ? would they only make you subnet from the classful nature, ie. 10.0.0.0 /10, 172.16.0.0/18 etc etc ?

Re: subnetting

Hey carl,

You might get, wheter the subnet is a valid one or not . So get ready for that too.

regards

vanesh k

New Member

Re: subnetting

So are we saying, if they tell me to subnet a 172.16.0.0 into /12 this is really a class A address ? even though by nature its a class B aadress ?

Re: subnetting

Hi ,

Pls donot get confused with Class B address and Class B network

class B address is one if the first two its are 10 and class B network is /16

If a question is asked , they would mention the prefix length along with adress and ask you to subnet like 172.0.0.0/8 into /12

hope it helps

regards

vanesh k

New Member

Re: subnetting

Can you expand on this explanation?

Re: subnetting

Hi,

The 32 bit ip addresses are divided into 5 classes , A,B,C,D,E --and each is one is identified by the first three bits of 32 bit

0 - Class A (values of first octet, 0 - 127 )

10 - Class B ( 128-191 )

11 - Class C (192-223 )

110 - Class D (224- 239 )

111 - Class E (240 - 255 )

But in terms of networks , three types

Class A : /8

Class B : /16

Class C : /14

Hope it helps

regards

vanesh k

New Member

Re: subnetting

would cisco ccna exam ask you yto subnet a 172.16 etc into a /8, how do they normally set the questions on the ccna exam ?

New Member

Re: subnetting

Please see the section "Number of Subnets: Subtract 2, or Not?" from http://www.ciscopress.com/content/downloads/cisco/1587200945appD.pdf

"Cisco makes no public statement about which to use if there are no hints in the question. In my humble opinion, if the question implies nothing at all about which formula to use, use 2n ? 2."

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