Subnet mask

[robertsmith10]

Hello,

    Can someone please tell me if you can calculate the subnet mask having only the ip and default gateway?

Thanks in advance.

[Karsten Iwen · Accepted solution]

As mentioned before, you can't tell that. But you can calculate the smallest subnet possible to make sure that both PC and DG are in the same subnet.

Example: PC has 10.0.0.194 and DG has 10.0.0.254. With a subnet-mask of /26 (255.255.255.192) both are in the same subnet. But with any bigger subnet (/25, /24, ...) the PC would also be able to reach it's gateway.

Challenge questions: Which minimum subnet is needed for:

1. PC 10.0.0.126, DG 10.0.0.129
2. PC 10.0.0.200, DG 10.0.1.254
3. PC 10.0.1.0, DG 10.0.1.255

[jamiebutterworth2016]

1. 10.0.0.0 /24

2. 10.0.0.0 /23

3. 10.0.0.0 /22

[acampbell]

Hi,

Basically you cant tell your mask unless you are using the class defaults from your info.

Class "A" = 255.0.0.0 (1.0.0.0 - 126.255.255.255)

Class "B" = 255.255.0.0 (128.0.0.0 - 191.255.255.255)

Class "C" = 255.255.255.0 (192.0.0.0 - 223.255.255.255)

Regards

Alex