Changing Subnet from 27 to 25. What all are precautions needed?

[Eranna]

Hi,

Need to correct the subnet value of the management network in a switch from /27 to /25. The management IP will be same only subnet mask is charging.

What all precautions need to take before execution. What will be the worst case scenario?

Thank you.

[Flavio Miranda]

@Eranna 

  Precaution is check on your network if the new /25 network, which is a bigger range, will not overlap with some other network you might have.

then, if this is a management network, you will need to update the mask in all network device that use this segment for management IP address.

If you have dynamic or even static route, make sure all of them will replect this new mask.

If you have firewall rule, make sure you update it accordingly.

The worst case scanario is you to break the traffic and lose access to devices  by not changing routing and firwall,accordingly.

[Joseph W. Doherty]

As you're expanding from a /27 to a /25, that encompasses the /25, the original /27 hosts should, I believe, continue to function still using the /27 mask. (I'm not suggesting any /27 masks not be changed.). What's at risk of not functioning correctly is the other /25 IPs that aren't within the original /27 scope.  I.e., it can be easy to miss one of the changes needed (like those @Flavio Miranda described) and scratch your head wondering why some of the /25 hosts (the original /27) work while others do not.

If you replaced the /27 with a non overlapping /25, you would get consistent behavior for the whole /25.

BTW, I'm not saying there's anything "wrong" with doing what you plan, just be aware that if you do miss some needed change, you may see different results based on actual host /25 IP.