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I have created 3 VLANS which should not overlap but somehow are giving me overlapping problems. Any reasons why? Is the switch buggy?

VLAN 10 10.1.1.0/28 255.255.255.240 Usable:10.1.1.0 to 10.1.1.15

VLAN 20 10.1.1.16/29 255.255.255.248 Usable:10.1.1.16 to 10.1.1.23

VLAN 30 10.1.1.24/28 255.255.255.240 Usable: 10.1.1.24 to 10.1.1.39

Switch(config-if)#int vlan10
Switch(config-if)#ip address 10.1.1.1 255.255.255.240
Switch(config-if)#int vlan 20
Switch(config-if)#ip address 10.1.1.17 255.255.255.248
Switch(config-if)#int vlan 30
Switch(config-if)#ip address 10.1.1.25 255.255.255.240
% 10.1.1.16 overlaps with Vlan20
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  • See the excellent answers to this question for how to calculate subnets and prefixes. Also, "VLSM" is an obsolete term from the classful networking era more than thirty years ago - it's called CIDR since.
    – Zac67 ♦
    Commented Mar 5 at 18:03

1 Answer 1

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The subnet that 10.1.1.25/28 belongs to is 10.1.1.16/28, which overlaps with VLAN 20. It doesn't magically start at .24 just because you want it to.

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  • I will need to use VLSM subnetting chart for this, correct?
    – Xavier Lim
    Commented Mar 5 at 20:12
  • See the comment above to your question and follow that link. You can use a chart or calculator -- up to you.
    – Ron Trunk
    Commented Mar 5 at 20:34

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