how much slack to allow while subnetting

hi all
i was wondering if theres any official guidlines (cisco,juniper , ietf etc) of how much space to leave when subnetting (for future growth)? i've searchd hi and low but cant seem to find anything official.
any ideas?

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GT-Rob

Its going to depend on your network requirements, and really the business requirements.

Generally, best practice is to use the smallest network size (so least amount of clients) that you can, while ensuring you won't run out in the future (which is impossible to predict!).

We actually use /16 networks on our user vlans, because we had more than 255 users per vlan (floor), and it was easier than teaching our guys how to VLSM lol. Basically said we need more than 255 per network, but we will never need more than 255 networks (we are on 10.x.x.x).

Our way is NOT best practice, but I guess was easiest practice at the time.

networker050184

I've never heard of anything official.

Firemarshalbill.com

I have never seen anything official, alot would be related to the businees model and expected growth of the company. You don't want to have 5% headroom when you are expecting 20% growth. Error on the side of too much space but not too much. I like to keep at least 20% overhead

GT-Rob

I will say the longer the network mask, the more flexibility you have with supernetting later. This is especially handy in networks with lots of remote sites with multiple networks behind each.