CIDR IP addressing

[Marcus Peck]

Hi experts,

     I have a scenario whereby I need to confirm my findings. I am given a Class A IP addressing range in the form of 10.21.28.0/24. I want to subnet this further and am safe to use the /27 subnet in this case. Results would be:

Usable	10.21.28.1 -	10.21.28.30
Host	10.21.28.0
Broadcast	10.21.28.31
Subnet Mask	255.255.255.224
Usable	10.21.28.33 -	10.21.28.62
Host	10.21.28.32
Broadcast	10.21.28.63
Subnet Mask	255.255.255.224
Usable	10.21.28.65 -	10.21.28.94
Host	10.21.28.64
Broadcast	10.21.28.95
Subnet Mask	255.255.255.224
Usable	10.21.28.97 -	10.21.28.126
Host	10.21.28.96
Broadcast	10.21.28.127
Subnet Mask	255.255.255.224
Usable	10.21.28.129 -	10.21.28.158
Host	10.21.28.128
Broadcast	10.21.28.159
Subnet Mask	255.255.255.224
Usable	10.21.28.161 -	10.21.28.190
Host	10.21.28.160
Broadcast	10.21.28.191
Subnet Mask	255.255.255.224
Usable	10.21.28.193 -	10.21.28.222
Host	10.21.28.192
Broadcast	10.21.28.223
Subnet Mask	255.255.255.224
Usable	10.21.28.225 -	10.21.28.254
Host	10.21.28.224
Broadcast	10.21.28.255
Subnet Mask	255.255.255.224

EAch of these will be assigned a VLAN and the GW for each is 10.21.28.0 or each host address? If this is in a switch connected to an edge firewall for internet, would the ip-default-gateway be the FW? IF there is an OSPF, can I just add 10.21.28.0/24 route in it? Thank you!                 

[Abzal · Accepted solution]

Hi,

One VLAN = One subnet. So in your case for each VLAN there will be different subnet /27 with own GW(e.g 10.21.28.1, 10.21.28.33 etc).

If it is L3 switch connected to FW you can assign IP address from other subnet /30 mask or create one more VLAN, assign IP address for VLAN then make access port connected to FW. And yes, it will be default GW. About OSPF the answer is yes, just put summarized subnet and it should be ok.

Hope it will help.

Best regards,
Abzal

[Marcus Peck]

Hi Abzal,

Thanks mate! The switch in question is a L3 switch.