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Beginner

Subnetting--> Simple Question

I'll rate the post of everyone who helps

 

So, the Network 10.0.0.0/8 was Subnetted into 2^19 Subnets. One of these Subnets, which is the 10.20.30.64/27 was Subnetted into 4 Subnets

 

I'm trying to find out the Network that the Subnet 195.3.40.0/21 came from. However, it's a Class C IP (/24), so how can it be with a /21 Mask?

 

- 10.0.0.0/8

                - 10.0.0.0/27

                - 10.0.0.32/27

                - 10.0.0.64/27

                …

                - 10.0.1.0/27

                …

                - 10.1.0.0/27

                …

                - 10.20.0.0/27

                …

                - 10.20.30.0/27

                - 10.20.30.32/27

                - 10.20.30.64/27

                               - 10.20.30.64/29

                               - 10.20.30.72/29

                               - 10.20.30.80/29

                               - 10.20.30.88/29

                -10.20.30.96/27

3 ACCEPTED SOLUTIONS

Accepted Solutions
Highlighted
Beginner

Re: Subnetting--> Simple Question

You can think of it as 0.0.0.0/0 is really the only IPv4 network out there, and it is sub-netted into various smaller divisions. Network classes are these days just a set of guidelines on how to divide it, unless you are working with very legacy software. Just as you can subnet a Class A network, /8 mask, into smaller parts, such as /27, a Class C network can be grouped with other class C networks to make larger ones, like a /21.

 

195.3.40.0/21 comes from the 0.0.0.0/0 network just like 10.0.0.0/8 does, unless you have some smaller network definition that you are working with in that space.

 

You might also look up supernet as a term used in contrast to subnet.

 

View solution in original post

Highlighted
Beginner

Re: Subnetting--> Simple Question

 

In the specific case of 172.16.100.0 is pretty complicated. What you can say about it is that 172.16.100.0 is only the address of a network if the netmask is /22 or above. At /21, 172.16.100.0 would be a host address in the 172.16.96.0/21 network rather than a network address.

 

I would answer as if the network address provided was a /24. Simply because only the last byte of the address is 0, and is a reasonable, and easy, assumption for mask given the information.

 

172.16.100.0/26,

172.16.100.64/26

172.16.100.128/26

172.16.100.192/26

 

 

It could also be answered based on a /22 boundary, so you end up with 172.16.100.0/24, 172.16.101.0/24 etc... But to work that out you need to be pretty quick with binary numbers or use a calculator.

 

I would hope test questions would either have the network addresses be valid for the normal classful netmask for the range, such as 172.16.0.0 being assumed as /16 or be explicit about the netmask. In fact I would hope that they would always be explict about the netmask, as nearly all software provides and requires explicit netmasks, there will be few real world situations where you have to guess at the netmask.

 

The one that can really trip people up is the 192.168.0.0 network. This is highly ambiguous between the private IP range of 192.168.0.0/16, and the first /24 network in that range of 192.168.0.0/24. It's very likely that ambiguity is why many switches and routers use 192.168.1.0/24 as their default network. The ambiguity is much less for the 172.16.0.0 network, as the two zero bytes tends to indicate /16 to match which the normal classful netmask in that range.

View solution in original post

Highlighted
VIP Expert

Re: Subnetting--> Simple Question

Are you doing classful or classless? Reason I ask, subnetting (and supernetting) are related to classful networking, and have very specific rules. When doing classless networking, we often refer to subnets, but all networks are "equal" (i.e. there are no subnets, although we do have summary networks).

So, for your 10.0.0.0/8 in classful networking it's a Class A, so anything greater than a /8 would be a subnet of it. If its classless, either you have the whole /8 network or it's a summary network.

For 192.168.2.0/24 in classful networking it's a Class C, so anything greater than a /24 would be a subnet of it. (192.168.2.0/23 would be a supernet.) If its classless, either you have the whole /24 network or it's a summary network.

View solution in original post

6 REPLIES 6
Highlighted
Beginner

Re: Subnetting--> Simple Question

You can think of it as 0.0.0.0/0 is really the only IPv4 network out there, and it is sub-netted into various smaller divisions. Network classes are these days just a set of guidelines on how to divide it, unless you are working with very legacy software. Just as you can subnet a Class A network, /8 mask, into smaller parts, such as /27, a Class C network can be grouped with other class C networks to make larger ones, like a /21.

 

195.3.40.0/21 comes from the 0.0.0.0/0 network just like 10.0.0.0/8 does, unless you have some smaller network definition that you are working with in that space.

 

You might also look up supernet as a term used in contrast to subnet.

 

View solution in original post

Highlighted
Beginner

Re: Subnetting--> Simple Question

Thanks for the help

So, Classless IPs can use any mask, they aren't attached to the Classful ones, however what about this

If an exam question asks me to Subnet 172.16.100.0 so that it fits 4 Subnets inside it and the Mask is not provided

Should I assume it's /16 (Class B IPs) or /12 (Class B Private IPs)?
Highlighted
Beginner

Re: Subnetting--> Simple Question

 

In the specific case of 172.16.100.0 is pretty complicated. What you can say about it is that 172.16.100.0 is only the address of a network if the netmask is /22 or above. At /21, 172.16.100.0 would be a host address in the 172.16.96.0/21 network rather than a network address.

 

I would answer as if the network address provided was a /24. Simply because only the last byte of the address is 0, and is a reasonable, and easy, assumption for mask given the information.

 

172.16.100.0/26,

172.16.100.64/26

172.16.100.128/26

172.16.100.192/26

 

 

It could also be answered based on a /22 boundary, so you end up with 172.16.100.0/24, 172.16.101.0/24 etc... But to work that out you need to be pretty quick with binary numbers or use a calculator.

 

I would hope test questions would either have the network addresses be valid for the normal classful netmask for the range, such as 172.16.0.0 being assumed as /16 or be explicit about the netmask. In fact I would hope that they would always be explict about the netmask, as nearly all software provides and requires explicit netmasks, there will be few real world situations where you have to guess at the netmask.

 

The one that can really trip people up is the 192.168.0.0 network. This is highly ambiguous between the private IP range of 192.168.0.0/16, and the first /24 network in that range of 192.168.0.0/24. It's very likely that ambiguity is why many switches and routers use 192.168.1.0/24 as their default network. The ambiguity is much less for the 172.16.0.0 network, as the two zero bytes tends to indicate /16 to match which the normal classful netmask in that range.

View solution in original post

Highlighted

Re: Subnetting--> Simple Question

It is not a Class C IP since Class C IP have default mask of /24 or 255.255.255.0. Rather, it is a Classless IP address.

To reduce the wastage of IP addresses in a block, we use sub-netting. What we do is that we use host id bits as net id bits of a classful IP address. We give the IP address and define the number of bits for mask along with it (usually followed by a ‘/’ symbol), like, 195.3.40.0/21. Here, subnet mask is found by putting the given number of bits out of 32 as 1, like, in the given address, we need to put 21 out of 32 bits as 1 and the rest as 0, and so, the subnet mask would be 255.255.248.0 or /21.
Highlighted
VIP Mentor

Re: Subnetting--> Simple Question

Hi

It can be originated from a summarization of networks, this network is supported under the interfaces and allowed into the routing table. 

 

Hope it is useful

:-)




>> Marcar como útil o contestado, si la respuesta resolvió la duda, esto ayuda a futuras consultas de otros miembros de la comunidad. <<
Highlighted
VIP Expert

Re: Subnetting--> Simple Question

Are you doing classful or classless? Reason I ask, subnetting (and supernetting) are related to classful networking, and have very specific rules. When doing classless networking, we often refer to subnets, but all networks are "equal" (i.e. there are no subnets, although we do have summary networks).

So, for your 10.0.0.0/8 in classful networking it's a Class A, so anything greater than a /8 would be a subnet of it. If its classless, either you have the whole /8 network or it's a summary network.

For 192.168.2.0/24 in classful networking it's a Class C, so anything greater than a /24 would be a subnet of it. (192.168.2.0/23 would be a supernet.) If its classless, either you have the whole /24 network or it's a summary network.

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