With a /25 how do I get a /30 out of it?

[Winston87]

Very new to networking and have been reading up quite a bit. Something that I am curious about is if I am given a /25 to put a network on and I want 4 ip addresses set aside as gateways for certain equipment how would that work out. Does that /25 turn into a /26 and a /30?

[Reza Sharifi · Accepted solution]

you have to break it down to multiple small subnets (/30s) for point to point interface and 1,  2  or 3 bigger subnet (/26 or 27s or 28s) for your hosts.

If you have a 10.10.10.0/25

you can get 16 /30 subnets and one /26 or 8 /29 and one /26 or 4 /28 and one /26

Here is an example:

IP Address       : 10.10.10.0
Address Class    : Classless /26
Network Address  : 10.10.10.0

Subnet Address   : 10.10.10.0
Subnet Mask      : 255.255.255.252
Subnet bit mask  : nnnnnnnn.nnnnnnnn.nnnnnnnn.nnsssshh
Subnet Bits      : 30
Host Bits        : 2
Possible Number of Subnets : 16
Hosts per Subnet : 2

Subnet Mask Inverse Mask Subnet Size Host Range Broadcast
10.10.10.0 255.255.255.252 0.0.0.3 2 10.10.10.1  to  10.10.10.2 10.10.10.3
10.10.10.4 255.255.255.252 0.0.0.3 2 10.10.10.5  to  10.10.10.6 10.10.10.7
10.10.10.8 255.255.255.252 0.0.0.3 2 10.10.10.9  to  10.10.10.10 10.10.10.11
10.10.10.12 255.255.255.252 0.0.0.3 2 10.10.10.13  to  10.10.10.14 10.10.10.15
10.10.10.16 255.255.255.252 0.0.0.3 2 10.10.10.17  to  10.10.10.18 10.10.10.19
10.10.10.20 255.255.255.252 0.0.0.3 2 10.10.10.21  to  10.10.10.22 10.10.10.23
10.10.10.24 255.255.255.252 0.0.0.3 2 10.10.10.25  to  10.10.10.26 10.10.10.27
10.10.10.28 255.255.255.252 0.0.0.3 2 10.10.10.29  to  10.10.10.30 10.10.10.31
10.10.10.32 255.255.255.252 0.0.0.3 2 10.10.10.33  to  10.10.10.34 10.10.10.35
10.10.10.36 255.255.255.252 0.0.0.3 2 10.10.10.37  to  10.10.10.38 10.10.10.39
10.10.10.40 255.255.255.252 0.0.0.3 2 10.10.10.41  to  10.10.10.42 10.10.10.43
10.10.10.44 255.255.255.252 0.0.0.3 2 10.10.10.45  to  10.10.10.46 10.10.10.47
10.10.10.48 255.255.255.252 0.0.0.3 2 10.10.10.49  to  10.10.10.50 10.10.10.51
10.10.10.52 255.255.255.252 0.0.0.3 2 10.10.10.53  to  10.10.10.54 10.10.10.55
10.10.10.56 255.255.255.252 0.0.0.3 2 10.10.10.57  to  10.10.10.58 10.10.10.59
10.10.10.60 255.255.255.252 0.0.0.3 2 10.10.10.61  to  10.10.10.62 10.10.10.63

IP Address       : 10.10.10.64
Address Class    : Classless /26
Network Address  : 10.10.10.64

Subnet Address   : 10.10.10.64
Subnet Mask      : 255.255.255.192
Subnet bit mask  : nnnnnnnn.nnnnnnnn.nnnnnnnn.nnhhhhhh
Subnet Bits      : 26
Host Bits        : 6
Possible Number of Subnets : 1
Hosts per Subnet : 62

[Winston87]

Ah yes that makes sense. Just started drawing blanks when I started looking at the subnetting. Thank you!

[Nagaraja Thanthry]

Hello,

You are correct. The /25 will become two /26 subnets. In one of them, you can further subnet to extract a /30 subnet.

Example: If we had 10.10.10.128/25 subnet, we could do 10.10.10.128/26 as one subnet 10.10.10.192/26 being the other /26 subnet. Out of .192 subnet, we could derive 10.10.10.192/30 as a /30 subnet. In this case, you will be left with few more possible subnets (.224/27, 208/28 and so on).

Hope this helps.

Regards,

NT

[Winston87]

Much appreciated. Thank you for also confirming that for me!