Hi @Ch Teo ,
Do a google search for subnetting without tears to find an easy way to think about subnets. The key to understanding subnets is that they only come in a limited number of sizes, each one twice the size of the previous one.
And of course, you also have to remember that every subnet needs a network address and a broadcast address, so the maximum host count is always 2 less than the subnet size.
| Mask (or prefix) length |
Subnet size |
maximum hosts=Subnet size-2 |
| /32 |
1 (2^0) |
NA |
| /31 |
2 (2^1) |
0 (unless using RFC 2031 which allows 2) |
| /30 |
4 (2^2) |
2 |
| /29 |
8 (2^3) |
6 |
| /28 |
16 (2^4) |
14 |
| /27 |
32 (2^5) |
30 |
| /26 |
64 (2^6) |
62 |
| /25 |
128 (2^(32-mask_length)=2^7) |
126 |
| /24 |
256 (2^(32-mask_length)=2^8) |
254 |
| /23 |
512 (2^(32-mask_length)=2^9) |
510 |
| /22 |
1024 (2^(32-mask_length)=2^10) |
1022 |
| ... |
... |
... |
| /2 |
2^30 |
2^30-2 |
| /1 |
2^31 |
2^31-1 |
So in your case, you have two subnets that won't quite fit into a /28 (size=16) so will have to have a /27 (size=32)
1. Sales: 26 host - needs a /27
2. Engineering: 16 host - needs a /27
3. Servers: 15 host (needs to be on a separate subnet) - needs a /27
4. Admin: 7 host - needs a /28
As to how to allocate these, imagine your original182.25.4.0/24 allocation is a pie. You can cut the pie in half, and continue cutting each half-pie piece in half and so on until you get the required number of slices of the right size. It is easier if you start with the biggest pieces. A search for rednectar's "IP Pie" will probably find you a powerpoint presentation I used to use that has an example.
So cutting the pie - starting with the biggest slices
Sales will take the first 32 IPs from 182.25.4.0 to 182.25.4.31 (/27)
Engineering will take the next 32 from 182.25.4.32 to 182.25.4.63 (/27)
Servers, the next 32 from 182.25.4.64 to 182.25.4.95 (/27)
Admin only needs 16 - so give them 182.25.4.96 to 182.25.4.111 (/28)
[Later] I note others have suggested that for "real world" applications, you may want to consider making the subnets a little bigger to allow for suture expansion. Although this is true, I'd like to point out that my answer was to the question asked - which was an assignment question. So to get full marks for your assignment (at least if I was marking it) you'd need to give an answer that has three /27 assignments and on one /28
RedNectar aka Chris Welsh.
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