One of my client sites called to ask me to change the subnet masks of the Linux servers I manage there while they re-IP/change the netmask of their network based on a 10.0.0.x scheme.

"Can you change the server netmasks from 255.255.255.0 to 255.240.0.0?"

You mean, 255.255.240.0?

"No, 255.240.0.0."

Are you sure you need that many IP addresses?

"Yeah, we never want to run out of IP addresses."

A quick check against the Subnet Cheat Sheet shows:

• a 255.255.255.0 netmask, a /24 provides 256 hosts. It's clear to see that an organization can exhaust that number of IP addresses.
• a 255.240.0.0 netmask, a /12 provides 1,048,576 hosts. This is a small < 200-user site. I doubt that they'd allocate more than 400 IP addresses.

I suggested something that provides fewer hosts, like a /22 or /21 (1024 and 2048 hosts, respectively), but was unable to give a specific reason against using the /12 subnet.

Is there anything this customer should be concerned about? Are there any specific reasons they shouldn't use such an incredibly large mask in their environment?