Ivan Petrushev's questions

Something weird happened today. We have a TP-LINK wifi router in our office distributing a pool of IPs 192.168.80-200. There are some windows and some linux boxes.

One of the linux boxes (CentOS) today booted up with IP 192.168.25.X.

I tried renewing the lease on my PC (Ubuntu) expecting to get 192.168.1.X, but here is what happened:

$ sudo dhclient r
$ sudo dhclient eth0
(long wait)
$ ifconfig eth0
inet addr:192.168.25.251

After a few minutes I renewed it again and it came with correct 192.168.1.81 (and from that point for now it is staying correct in every re-newal)
Where from that another network came up?
Later I remembered I could add -v to dhclient to see some more verbose info especially which DHCP server is responded to my request.

Could it be another DHCP server in our network I'm not aware of? I tried looking in /var/lib/dhcp leases but couldn't find traces of the 25th network.

It seems the windows PCs are not affected, only the linux boxes.

I have the following situation and I don't know how to ask google.

I have a cabled LAN segment, lets say 192.168.1.X. One of the nodes there is a wifi router 192.168.1.3 that creates wireless segment 192.168.2.X for some laptops.

I want to put all of the computers in the same network (192.168.1.X) and not to maintain 2 different networks (1.X and 2.X). I don't want to have a gateway or firewall between them. Is this possible? What is this mode called?