I was looking into VLANs recently and noticed that in several tutorials, VLAN tags with large numeric gaps were chosen, like 10, 20, 30, etc. Is there a reason why one wouldn't simply enumerate it like 1, 2, 3, 4, ...?
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It does not matter, but leaving gaps between enumerated things, e.g. ACL lines, is a good practice, and people often carry that over to other things.
You may want to leave room to add logically related VLANs to be in the same range of 10 to 19. For example, if you have department VLAN numbering, and a department that uses VLAN 10 for its data wants to add a VoIP VLAN, then you can make that VLAN 11.
Not really an official best practice, and I know this is primarily opinion (which is frowned upon here) but, like Ron said already, leaving space between VLAN IDs allows for better grouping of similar types of devices and such.
Additionally, matching the VLAN IDs to the third octet of an IPv4 network makes it easy to keep track of which VLAN is associated with which network.
VLAN 10: 10.10.10.0/24 VLAN 11: 10.10.11.0/24 VLAN 12: 10.10.12.0/24
The reason I prefer doing this on the third octet is mainly due to route summarization. If the VLAN IDs were matched networks on the second octet, for example, you’d have to include each network separately in route advertisements or static routes.
Take this example: With 10.10.10.0/24, 10.10.11.0/24, and 10.10.12.0/24, assuming that all networks (present and future) with 10.10.x.x will reside at the same physical location, I can summarize my route advertisement as 10.10.0.0/16 and any new networks I may create within the environment will automatically be included in the summary route.
If I used the second octet instead, there is no clean way to summarize the routes.
Take this example: With 10.10.0.0/24, 10.11.0.0/24, and 10.12.0.0/24, you would have to advertise them as 10.10.0.0/15 and then 10.12.0.0/24, or figure out how large you want to go, and then also figure out what you would do for other locations without overlap or using a completely different scheme. It gets really messy.