/16 Subnet doubts.

Edgar Veloz
Edgar Veloz used Ask the Experts™
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I have a customer that recently rebuild their DHCP, they had a /24 subnet and now they are /16 they have around 300 devices registered and everything for the most part is working fine, but I have a a blue alert on the DHCP and I dig into the properties of the LAN I see that start IP Address is 10.11.0.1 and End IP Address is 10.11.254.254.

To my knowledge a blue alert means you're running out or ran out of leases but they should have enough addresses but maybeI'm wrong.

Is this correct ? DHCP Scopes are not my strongest topic I think is ok but I could definitely being wrong.

Any assistance would be really appreciated it.
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The last usable in 10.11.0.0/16 would be 10.11.255.254. So, you were close! Within the DHCP pool there is no requirement to allocate the whole /16 so there is nothing wrong with the pool ending at 10.11.254.254.

Nevertheless, that's a whole lotta IP addresses! Are client devices not getting IP addresses from the server?
Frankly, for IP management's sake, I think I'd shrink that pool a bit as it's highly unlikely the pool needs to offer 65,000 IP addresses. You can shrink the pool in CIDR notation or just as easily making it end at 10.11.3.254 or wherever you think would make sense.
Dr. KlahnPrincipal Software Engineer

Commented:
Going from 255 addresses (/24) to 65,536 (/16) is one big jump.  It would perhaps be prudent per Matty's suggestion above to use a /20 or /22 for starters unless there is a pressing reason to make the jump.  That way if there is need to partition later on, whoever has that unenviable job will not be saying "Move these out of here, move them to here, repartition at /19 for these offices, etc. etc."
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Edgar VelozIT Consultant

Author

Commented:
I agree with you @Matty-CT I just came in, into this project to work something non-related to this, I'm migrating them to Azure and O365 and while looking into the local DHCP I ran into this, so I thought to ask, but yes I'm going to make the recommendation to shrink the pool to probably 10.11.4.254

Thanks Guys.
Distinguished Expert 2018

Commented:
Blue exclamation can also happen if the scope or settings in the scope sre disabled. It isn't always representative of an exhasuted pool.
yellow exclamation mark means you’re almost out of IPs.

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