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We have a router with NAT enabled on our edge. The NAT inside is 10.0.0.0/24. Now, we're running out IP blocks and we wish to change this to 10.0.0.0/22. We can do this quickly on the NAT router. But for the host devices, do we need to replace their subnet mask as well and use /22?

We have a mix of DHCP and Static IP address users here. My initial thought is we can skip them from changing the Subnet Mask and still use /24 since the default gateway is still 10.0.0.1. Is this a correct assumption? Thanks.

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    A /22 is rather large for a single broadcast domain. Have you considered dividing the LAN into VLANs? you could then have four /24 VLANs from the /22, and add other VLANs as needed in the future.
    – Ron Maupin
    Sep 14 '15 at 14:14
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin
    Jan 4 at 0:39
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Yes, you should change the netmask to /22 on all the devices. While some things may work, there is a lot of potential for problems, especially when internal devices talk to each other. It may seem like a PITA, but changing the netmask will save you lots of late night troubleshooting later on.

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  • This is also my concern. Technically a /22 can cover those in a /24 but there are probably unforeseen effects.
    – Marc_A
    Sep 14 '15 at 16:08
  • Note that you can switch the NAT (and all DHCP devices) to /22 and then take some time to reconfigure the static devices. Any ongoing communication won't be disrupted. New connections from unreconfigured static devices through the NAT will continue to work. Problems will occur only for local-to-local communications, and then, only when a static device tries to connect with a DHCP device that has obtained a new lease that's no longer on the old /24 subnet. Jun 11 '18 at 20:16
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If it's not a requirement for all the devices to be on the same subnet, and your hardware supports it, you could just configure adjacent /24s on the same interface. You CAN just let the /22 and /24 overlap, but as others have said it's not recommended.

Cisco would be something like:

interface GigabitEthernet0/1
 no ip redirects (optional)
 ip addr 10.0.0.1 255.255.255.0 
 ip addr 10.0.1.1 255.255.255.0 secondary
end

Note with this configuration, traffic routing between these subnets would have to "hairpin" or go up to the router and come right back out the same interface, taking away from the available throughput on the link for outside traffic. This is the behavior of "no ip redirects" which I always like to add because not all OSes handle ICMP redirects gracefully and it doesn't scale well (not that my suggested method does anyway).

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Dividing them into VLANs of /24 is the correct answer. The easy route is to use /22 on a flat network, but that's using a single Broadcast domain. If there's one NIC that fail and act up, all users will be affected.

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