Your router can indeed lease that IP address to another client once the lease expires if your machine is off or disconnected, and doesn't renew it. To guarantee you always get the same IP address, you'll want to make what's called a DHCP Reservation in your router (terminology may vary - see this question for details):
With DD-WRT firmware, you can find this under the Services section:
It looks like you have a new DHCP server on your network.
The first thing you should try and see is which gateway you are assigned, when you are on the bogus subnet. If it is in the range 192.168.1.0/24, you may probably guess on which computer it resides, since you stated your pcs are all on static IPs.
Or, you may try turning off your pcs one by one, and see when your wireless devices, eventually, acquire an IP address in the 192.168.1.0/24 range. The last pc turned off is then the culprit.
Or, you may use a Ubuntu installation (drop an Ubuntu image onto a USB stick, and boot from that)to issue the following commands:
sudo service network-manager stop
dhcp -v eth0
from an ethernet-connected pc. The output,
dhclient -v
Internet Systems Consortium DHCP Client 4.2.4
Copyright 2004-2012 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Listening on LPF/eth0/e8:e0:b7:be:72:6a
Sending on LPF/eth0/e8:e0:b7:be:72:6a
Sending on Socket/fallback
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 3 (xid=0x333a3d1d)
DHCPREQUEST of 192.168.73.88 on eth0 to 255.255.255.255 port 67 (xid=0x333a3d1d)
DHCPOFFER of 192.168.73.88 from 192.168.73.1
DHCPACK of 192.168.73.88 from 192.168.73.1
bound to 192.168.73.88 -- renewal in 35456 seconds.
shows this critical line,
DHCPOFFER of 192.168.73.88 from 192.168.73.1
which displays the IP address of the pc making the offer (192.168.73.1, in my case).
Or, assuming these IP addresses don't mean a thing (i.e., they fall in the 10.0.0.0/8 range), you may try installing a program called nmap, and run it with the command
nmap -T4 -A 10.55.142.1
where 10.55.142.1 is the gateway your pc is assigned, if it is not 10.55.142.1 please change it to its true value. This will tell you many things about the host pc, including (if you only have Windows machines) its name.
This should at least identify the pc where the new DHCP server is running.
Best Answer
Page 187 of the X3500 manual shows how to extend the range of the DHCP server. Generally speaking, you might want to extend the subnet of your DHCPs range. What does your configuration look like? How many clients do you actually have on your network?
Since the manual says that 192.168.1.1 is the default IP adress of the router, take the following example on how your network might be configured:
This will net you 1022 total available IPs for DHCP. Furthermore, you might try to reduce the DHCP lease time. This will make your router reassign IPs faster.