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Hei,

Suppose we have 192.168.0.0/24 and we subnet it as 192.168.0.0/25. Is it a normal thing to configure hosts with IP addresses in both /24 and /25 address spaces ? We consider /24 and /25 to be in different VLAN.

Thanks

Iasmina
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  • As long as they're isolated from each other I think it's not a problem - otherwise you could have address conflicts. – manish ma Apr 17 '23 at 12:55
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    An IP address is a 32-bit number. It does not matter how you subnet it, the number will be the same number, so you will have multiple addresses that are the same, and you cannot do that unless you NAT between the networks. – Ron Maupin Apr 17 '23 at 13:09
  • Unless you're talking about proxy-arp (and you shouldn't!), the minute you subnet to /25 (etc.), the /24 no longer exists as a LAN. – Ricky Apr 17 '23 at 21:32

2 Answers2

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Is it a normal thing to configure hosts with IP addresses in both /24 and /25 address spaces ?

No. Having overlapping subnets disrupts communication. If you subnet 192.168.0.0/24 in two, you get 192.168.0.0/25 and 192.168.0.128/25.

When running 192.168.0.0/24 and 192.168.0.0/25 in the same broadcast domain/VLAN, hosts from the /24 prefix can send to the /25 nodes fine, but the /25 nodes cannot send to IP adresses 192.168.0.127 through .255 (which they'd try to use a gateway for).

IP addresses need to be unique at all times, regardless of prefix length. Duplicate addresses make routing ambiguous and break your network. You can only get around that by using NAT. However, NAT is an ugly kludge because it breaks IP's end-to-end paradigm.

We consider /24 and /25 to be in different VLAN.

The hosts from 192.168.0.0/25 would use a gateway to reach 192.168.0.128-254/24, but the reverse and everything else would break.

This Q&A should be able to tell you all about subnetting.

Zac67
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You are creating problems for yourself. IP addresses are always unique - regardless of the subnet mask.

If you have 192.168.0.0/24 on VLAN 1, and 192.168.0.0/25 on VLAN 2, then all traffic in the range 192.168.0.0 - .127 will be routed to VLAN 2. If you have hosts on VLAN 1 in that range, they will be unreachable.

Hosts in VLAN 1 will never be able to talk to hosts in VLAN 2 because they assume the destination is on the same VLAN.

Ron Trunk
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