I’ve run into a situation a few times where, for some unknown reason, a Windows machine will be unable to obtain an IP address via DHCP.

Then that happens, the machine is likely to get an APIPA address, such as 169.254.xxx.xxx. This is known as the DHCP failover situation, and generally, it’ll mean that while you might be able to see sites out on the internet, most likely, you won’t be able to see any computers connected to your internal network (often computers on your internal network will be associated with IP addresses of 192.168.xxx.xxx).

In my case, I could set static addresses for those affected computers and they’d work fine, but reset them back to using DHCP and they’d be unable to obtain an IP address.

Googling turned up a variety of possible solutions (unplug your router, reset the IP stack by deleting registry entries, plug your computer into a different power outlet <huh?>, running IPCONFIG /RELEASE….IPCONFIG /RENEW, etc), but nothing was working for me.

On a lark, I browsed over to my router (in this case a Verizon Fios router) to check it’s configuration.

I have several routers, but the Verizon router is the only one set up to be a DHCP server (there should only be one machine, router, or actual server, on your internal network set up as a DHCP server), and it all looked good, but then I noticed something.

I had a LOT of devices showing up in the connected devices list. Many were offline, but they were still there.

This means that the router was still holding leases for them and tying up those IP addresses. Hmmmm.

I counted. 21 of them.

So I navigated to the DHCP configuration screen of the router and lo and behold, I had it set to lease addresses from 192.168.100.100 to 192.168.100.120. Exactly 21 addresses!

Ugh.

Why?

Way back when I set up this router, my thinking was “I’ll never have more than 20 devices connecting, so why should I accommodate any more than that?” so I’d limited to the available IP address range that the router would pick from.

In retrospect, I suppose that was being a little to prematurely security-conscience.

It was that limit, along with a router than apparently does not like to recycle IP leases readily, that caused me to simply run out of DHCP addresses, and thus the failure of Windows to acquire an IP address.

Moral of the Story

At the end of the day, I probably shouldn’t have limited the router to 21 addresses. It seemed like a reasonable number, but these days, with several computers in each house, plus smart phones, TIVOs, internet-connected TVs, IPADs, laptops, game systems, etc, it’s completely conceivable to have far more than 20 devices or so vying for IP addresses through DHCP.

When I exhaust the range from 192.168.100.100 to 192.168.100.200, then I’ll know I have too much tech <g>!