Subnet mask /23 vs /24

[ranga2002]

Hi SMEs,

This question is a simple one regarding subnet mask. There was a server in the network and had to replace the server with a new one.(one to one replacement- Disconnect the old one from the network and connect the new one)

Existing IP: 10.10.1.20
Subnet Mask : 255.255.254.0 (/23)
Gateway: 10.10.1.254

However, when I replaced it with the same IP information, it was not pingable. IT worked only after I changed the subnet mask to /24.

New IP : 10.10.1.20
Subnet Mask : 255.255.255.0 (/24)
Gateway : 10.10.1.254

My question is, how it worked earlier with the/23 mask and how come /24 resolved the issue? Why didn't the new server didn't accept /23 mask, which was working.

[balaji.bandi]

Was seen the same issue, is this windows server 2009 or 2012 R2 ? As VM and Physical Server.

If your Gateway is originally set up as /24 network as Gateway, Old version doesn't care I guess the network and broadcast, used to work.

when the new Server 2016 or 2019 not going to accept that. so you need to be the same mask and part of the broadcast domain to work.

[MHM Cisco World]

good point thanks.

[ranga2002]

It was a linux server.

[balaji.bandi]

give more information, what Linux was this ? 

[Richard Burts]

I find this description puzzling. If I am reading the post correctly both the server and the gateway were in the same subnet with both masks /23 and /24. If the new server works with /24 and does not work with /23 then it must be something in the OS of the server. And I thought linux was pretty good about subnet masking. I have seen some issues (especially in Windows) when the addressing was using supernetting. But that is not the case here.

If the server is configured with 10.10.1.20/24 it would recognize the upper half of the /23 as local and regard the lower half of the range as remote. Does this impact connectivity between hosts in the lower half and the server?

[ranga2002]

Hi Richard,

Once the new server configured with /23 (as same as the existing server), it was not responding to ping or ssh from remotely and locally. Yes, I agree it is strange.

[Richard Burts]

I want to focus just on the server and the gateway and to verify that if the server is configured with 10.10.1.20 and gateway of 10.10.1.254 and mask of 255.255.254.0 the the server is not able to ping the gateway and the gateway is not able to ping the server? Could you test it again and post the content of the arp entries on both the server and the gateway?

[ranga2002]

Hi Richard,

Unfortunately, I coudn't test the arp entries as it was totally unreachable. But I'll make sure to try it out if in future I face the same issue. Thanks all for your time to help me out.

[BalanceDove]

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@Richard Burts wrote: DGCustomerFirst

I want to focus just on the server and the gateway and to verify that if the server is configured with 10.10.1.20 and gateway of 10.10.1.254 and mask of 255.255.254.0 the the server is not able to ping the gateway and the gateway is not able to ping the server? Could you test it again and post the content of the arp entries on both the server and the gateway?

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