Thursday, April 17, 2008

Where have all the IPs gone.... all the wireless users, of course ! Several weeks ago I went
looking for IP subnets to assign to our new wireless SSIDs. What I
discovered is that we do NOT have the vast amounts of unused IP space
we once thought.

But first things first... the first step was to move our IP allocation
documentation from spreadsheets and flat files into a database.
Fortunately, we already developed a nice IP allocation database for
our support of networks like Internet2 and NLR, so we had a database
ready to go. Now that all our IP allocations for all our campuses
are documented in a single place, we can look at overall IP
utilization, delegate authorization to allocate addresses from
specific IP blocks, and do better planning of our IP allocations.
This will become very important as our IPv4 address space becomes more
scarce !

Once I started looking at this, I found that, especially in
Bloomington, we don't have a whole lot unused subnets and especially
not contiguous subnets. And it turns out a LOT of these are eaten up
by wireless users !

According to our monitoring software, we are seeing about 5,000
simultaneous wireless users in Bloomington these days. However, our
DHCP lease timers are in the 90-120 minute range [see note below].
So if someone uses wireless for 10 minutes and then shuts their
laptop, their IP address is reserved for another 80-110 minutes.
This means we actually have about 10,000 total host IP addresses
assigned to our wireless subnets ! That's 1/6th of a whole /16 or
legacy Class B block. But it gets worse !! Since users must use
VPN to get full access to wireless, most of these users are also
consuming an IP in the VPN address pool. So we have several thousand
more IPs assigned to those pools for a total of nearly 16,000 host IPs
assigned for wireless users. That's 1/4th of an entire /16 or Class
B !!!

Note: On DHCP lease timers, we'd love to decrease them, but there's an
issue with some VPN clients that, when they have a VPN connection,
they don't renew their lease properly because they send DHCP packets
improperly over the VPN tunnel instead of to their local subnet, so
when their DHCP lease expires they loose their network connection
until the VPN tunnel drops and they renew their lease over their local
subnet. We used to have shorter lease times, but many users
complained that their VPN connections kept dropping in the middle of
meetings and they would have to reconnect. This won't be an issue on
the new WPA2 Enterprise SSID !

Even with shorter lease times on the WPA2 Enterprise network, given
the level of growth we're seeing in wireless usage and all the new
wireless clients from the expansion into the dorms, we think we need
to at least allocate 16,000 host IPs to the new wireless network.
Since we can't reclaim the IP space from the current wireless network
until users transition to the new one, we need to come up with a new /
18 block of IPs. The *ONLY* block we can take this from is which is the last unused Class B network we have.
Since we've never used this block, we need to give ample warning to
all system administrators incase they have host firewalls that need to
be updated. And THAT, my friends, is at the top of my to-do list for
today !

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