Unless someone screwed it up, there's no way in hell it has DHCP. There's some other server or machine on your network or maybe someone hooked up a wireless router so they can surf on their iphone and didn't turn off the DHCP.
How do you have your network configured? You do have a switch and/or router connected to the 851 right? If you plug any machine directly into the 851s 4 ports you'll get nothing. Your router's WAN port goes to one of it's LAN ports and you go into the router's setup page and change it to static and type in your address block and gateway.
I have 2 modems at work (technically 3, one is just voice) and 2 851s and neither of the 2 provide any DHCP service on their LAN ports.
That is to say, the router itself if you buy it from Cisco and use it as your own does behave normally as a router with all the standard Cisco iOS stuff. It however doesn't have any of it's interfaces enabled when it's used with Optimum Online Business is what I meant to say. So if you grabbed the manual for it online, then sure it will say it does DHCP. OOL doesn't use it that setup however.
