I am a little confused with assigning ip to computers. I have a topology like this : enter image description here

Should all segments between Router 9 and Router 10 be on the same subnet ? I should be able to communicate between vlans, so I have to create sub-interfaces on Router 9, but to do this, I have non-overlapping ip for each vlan. If vlans should be on different subnets, what with these two segments with VLAN 30 ?

I am confident, that it might be basic question, but I don't have much experience. Could someone give me any hint ? What is the good way to address it ?

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    Aug 11, 2017 at 3:31

2 Answers 2


The picture specifically states that the R9-S0 and S0-S1 links are trunks, while PC0 and R10 are only on vlan 30 and PC1 is on vlan 20. Since the diagram shows two different vlans, you would normally use two different subnets.

  • quote cpt_fink: only different vlan shall be on different segment: one virtual interface of router 9, PC0 and the interface of router 10 that connect to Switch1 must be on the same network (to communicate each other on vlan10). The trunk link, instead, can bring different vlan, so on them there are (in this case) 2 different Vlan (so 2 different subnet), plus the native vlan (but that's another story ;).
    – feligiotti
    Apr 25, 2015 at 12:40

If you want to divide your network by VLANs each one needs is own subnet. Here is a link to some Cisco documentation http://www.cisco.com/c/en/us/td/docs/ios/12_2/switch/configuration/guide/fswtch_c/xcfvl.html

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