subnetting a network

[davideire12666910]

Hi all, 

I have a college assignment on cisco packet tracer networking and for additional marks the lecturer is asking that we can try subnetting , now i figured out how to do it, but I'm not sure if I'm following best practices,

so its a college network with ip address of 192.168.0.0/24(college network)

and then we are given different ip for different departments, 192.168.1.0(Admin), 192.168.2.0(Accounts), 192.168.3.0(lecturer),etc 

and in each department there is like 20 /50 host , now what i have done its taking the 192.168.0.0 and subnet every ip from there, and my question is : is that good practice ? or should i have sub netted the 192.128.1.0 for the host need it? so when sub netting are we taking the global 1st address or the individual networks address provided, i hope i made the scenario clear, sorry absolute newbie here.

thanks in advance

David

[Georg Pauwen · Accepted solution]

Hello,

it is a bit unclear what the IP address assignment is supposed to be.

192.168.0.0/24 gives you the host range 192.168.0.1 - 192.168.0.254. You cannot subnet this into networks 192.168.1.0/x, 192.168.2.0/x, and 192.168.3.0/x.

The following would give you 62 hosts:

Subnet ID: 192.168.1.0
Subnet Mask: 255.255.255.192
Host Address Range: 192.168.1.1 - 192.168.1.62

Subnet ID: 192.168.2.0
Subnet Mask: 255.255.255.192
Host Address Range: 192.168.2.1 - 192.168.2.62

Subnet ID: 192.168.3.0
Subnet Mask: 255.255.255.192
Host Address Range: 192.168.3.1 - 192.168.2.62

If you need 20 hosts, you need to change the subnet mask:

Subnet ID: 192.168.1.0
Subnet Mask: 255.255.255.224
Host Address Range: 192.168.1.1 - 192.168.1.30

Subnet ID: 192.168.2.0
Subnet Mask: 255.255.255.224
Host Address Range: 192.168.2.1 - 192.168.2.30

Subnet ID: 192.168.3.0
Subnet Mask: 255.255.255.224
Host Address Range: 192.168.3.1 - 192.168.2.30

[davideire12666910]

No it makes sense, something so stupid but I was so confused thanks a lot mate,

[Joseph W. Doherty]

Your question is a bit unclear because you can "subnet" 192.168.0.0/24 and/or 192.168.0.1/24, 192.168.2.0/24 and 192.168.3.0/24.

BTW, although we still use the term "subnetting", since "classful" IP is a pretty much defunct (with "classless"), you create networks pretty much however you want, provided they are on a power of two address boundary.  For example, you might have networks 192.168.0.0/23, 192.168.0.0/25 or 192.168.0.0/31.  How they differ, is the address block/space they consume.

Best practice is to not "waste" address space, both in using an address block too large for your needs or using networks that preclude using other address blocks.

An example of the first case, as a /24 has 256 IPs, if you know for sure you only know you need to deal with up to 50 hosts, then an address block that provides 64 IPs (or 62 host IPs), i.e. /26, would normally be a better (best practice) address block allocation.

For the second case, if you allocate 192.168.1.0/26, 192.168.2.0/26 or 192.168.3.0/26 (as shown by Georg), you can no longer allocate 192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24 or 192.168.2.0/23.  I.e. you're stuck if you also need a larger address block (assuming you only have 192.168.0.0/22 to work with).

So, if you needed four /26 blocks, what would be better would be using 192.168.0.0/26, 192.168.64.0/26, 192.168.128.0/26 and 192.168.192.0/26 leaving 192.168.1.0/24 and 192.168.2.0/23 untouched.

So, again, the two goals of address space allocation is to not waste it either by providing a address block too large for what's needed and/or allocating address blocks such that they preclude using other larger address blocks.

[davideire12666910]

Thank you very helpful answer, its definitely giving me a better understanding of the subject.