VLSM query

[John Cheetley]

Hi cisco techs,

Am learning VLSM for  10.1.10.0 address. As I have seen. There is many tutes for B and C but not explicitly for A

The below requirement is needed

120 users for 1st allotment

31 users for 2nd allotment

4 for 3rd allotment

4 for 4th allotment

Magic number is 128 in line with binary 2 to the power table

Subnet mask becomes 255.255.255.11111111 (so therefore all the bits are used in the last octet)

Network address becomes..

10.1.10.0

Allotment of hosts for 1st allotment is 10.1.10.1 - 10.127 (for 120 users with 6 ip's not being used)

How are the last 3 allotments completed when the last octet has been used or unless the last 3 use the rest of the allocation up to 254?

Thanks again

[Joseph W. Doherty]

Am learning VLSM for  10.1.10.0 address. As I have seen. There is many tutes for B and C but not explicitly for A

Unclear what you're learning, or the rules that apply to classful B and C addresses.  (The only one that comes to my mind, is supernetting of classful B and C addresses, while class A addresses aren't generally supernetted.)

Assuming you're using CIDR, there's not much need to subnet classful networks.

Using CIDR, your four cases could be met using a /25 a /26 (or /27 if we also use the network address as a user address), and a /29 and /29.

Also unclear what your "10.1.10.0 addresses" encompass.  I'm guessing, perhaps you mean to use the 10.1.10.0/24 address block.  If so, it could be subdivided to provide a /25, /26, /29 and /29.

Allotment of hosts for 1st allotment is 10.1.10.1 - 10.127 (for 120 users with 6 ip's not being used)

I think you meant 10.1.10.1 - 10.1.10.127.

If so, the /26 could use 10.1.10.129 - 10.1.10.190.

The two /29s could use 10.1.10.193 - 10.1.10.198 and 10.1.10.201 - 10.1.10.206

[John Cheetley]

Thanks Joseph for your input

So no supernetting for class A addresses.

So how do the other 3 allotments use the block size of 128 (as the first allotment does) please?

[Joseph W. Doherty · Accepted solution]

So no supernetting for class A addresses.

Actually you could, but the only reason I could see for doing that is for very high level route aggregation.

So how do the other 3 allotments use the block size of 128 (as the first allotment does) please?

Depends on what your initial block is.  If it were a /24, you wouldn't be able to allocate four /25s.  If it were a /23 you could allocate 10.1.10.0/25, 10.1.10.128/25,10.1.11.0/25 and 10.1.11.128/25; each providing 128 addresses.

[John Cheetley]

Original allocations as per below

120 users for 1st allotment (I used 128 block for this)

31 users for 2nd allotment (was going to use 32 but wasn't sure if could and confused for the rest)

4 users for 3rd allotment

4 users for 4th allotment

[John Cheetley]

Unless you can use different blocks for different allotments?

[Joseph W. Doherty · Accepted solution]

Often you can.  I assume that was the case in my first post.