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I have a 255.255.255.0 subnet mask (/24) can I change the subnet mask to 255.255.254.0 (/23) and not affect the hosts? The hosts are hard coded with no DHCP in this scope. Will I have to change the network settings for each host after I change this?

  • Changing the network mask means you change the masks on all the host interfaces (router, PCs, printers, etc.). There is no single network mask; the network mask on a host describes the network from the perspective of the host. Also, you have explained the network address, so it will either include the previous or next /24 network in the current network, depending on the 24th bit of the.current network (0 would add the next /24 network, and 1 would add the previous /24 network). – Ron Maupin Jul 22 '19 at 22:48
  • So since the hosts (100+ hosts) are all hard coded with 255.255.255.0 I would have to change them all to 255.255.254.0? We are basically all out of addresses on the /24 ... Creating a new subnet is not possible given the way the switches are configured. – rjmend Jul 22 '19 at 23:01
  • What constitutes a network for a host is based on its configured address and mask. If that is statically configured, then nothing changed outside the host changes what the network for the host is. – Ron Maupin Jul 22 '19 at 23:07
  • "Creating a new subnet is not possible given the way the switches are configured." Switches care nothing about subnets or IP addresses. – Ron Maupin Jul 22 '19 at 23:09
  • @RonMaupin sure if you are talking about the rare switch that is actually only a switch. These days most switches in business supports some layer3+ features. – Zoredache Jul 22 '19 at 23:20
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The purpose of the prefix (and the deprecated netmask) is to define which part of the address space is on link. If a host has the wrong prefix configured, it will not be able to reach all of the addresses on that link (example). So you will need to reconfigure every host which has had its address and prefix manually configured. I recommend you take this opportunity to undo the mistakes of the past and institute some sort of centralized IP address management, or at least a DHCP server.

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