VLSM Error on Cisco Distribution Switch

Question

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

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, Mar 05 '24 at 18:03

score 2 · Answer 1 · answered Mar 05 '24 at 17:29

2

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.

answered Mar 05 '24 at 17:29

Ron Trunk

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I will need to use VLSM subnetting chart for this, correct? – Xavier Lim Mar 05 '24 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 Mar 05 '24 at 20:34

VLSM Error on Cisco Distribution Switch

1 Answers1