Local hostname already in use - can't figure out what's going on

Question

I've been having issues with a wireless router and I think the problem lies somewhere within my computer. As I switch back and forth from trying to connect to the internet via my router or directly connected to my cable modem, I will often get this error:
"This computer's local hostname "xyz" is already in use on this network"
There is only one compuer on the network, however and I cannot figure out what's going on.
I have 2 users set up on this machine, but I'm the admin and acting as such. Even with no other networked equipment hooked up to my Mac (except for the cable modem) this problem won't go away.
I guess my real quesiton is what causes this and where should I go to troubleshoot things?
I'm running 10.4.6, connect to my ISP using DHCP and have the same problem with both a D-Link and Linksys router.
Any ideas?
Thanks!

Answer 1

Thanks very much for your help, fu.
If your previous setup follows a flow like this: RCA
CableModem-->NetGear Router-->AirPortExpress, there's
something misconfigured routing-wise in your network.
You have to decide which device (Netgear or AirPort
Xpress) will handle the routing in your network.
Previous to these problems, that was the basic flow of my network. The netgear router also fed a wired connection into a Wintel PC, and the AirPort Express hosted a wireless network that was extended by a second Airport Express. And that second AE connected to a ReplayTV with a wired connection. And then, of course, the PowerBook got Internet access wirelessly through the AE-hosted network. And that was it.
That network always was configured so that the netgear router assigned IP addresses using DHCP, and the Airport Expresses would not assign IP addresses (i.e., "Distribute IP addresses" was NOT checked when you look at the configuration in Airport Admin Utility).
How big is your network j? just 1 Mac (your
PowerBook)?
That was the state of the network a couple of days ago when I posted this. I have established that the PowerBook gets access to the Internet just fine directly though the cable modem, and just fine when it is the only wired connection to the router.
Let's deal with your Mac first:
Start with your Mac connected directly to the cable
Modem. (When your mac is attached directly to the
Cable Modem, do you use DHCP?)
Go to SystemPreferences-->Sharing and type a Computer
Name other than the default one
Open Applications-->Utilities-->Terminal and type
lookupd -flushcache (press Return) and reboot
the Mac.
I did this.
One way to fix your network is to have the NetGear do
the routing, and the AirPort Express to (just) extend
your (wired) network wirelessly (No routing will be
done from the AirPortXPress, (No distribution of IP
Addresses)
I soft reset both of my Airport Expresses. I configured the first one so that it was hosting a network, but not distributing IP addresses, and the second so that it was extending that wireless network. And I plugged the host AE into the netgear router. And for a little while, that was working.
But then I got a message saying "IP Configuration 192.168.0.4 in use by 00:04:40:06:1f:82, DHCP Server 192.168.0.1". The dialog box with that message will come up periodically as long as the Airport Expresses are plugged into power and one is plugged into the router. Also, I once agaiin received the message saying the local hostname (the completely original one that I had created) was in use, and the name was being changed.
Here is the thing. I cannot figure out what has that ...0.4 IP address and ...1f:82 MAC address on the network. That is not the "Internet port MAC address" or the "LAN port MAC address" for the Netgear router. It is also not the MAC address for either of the Airport Expresses or the PowerBook's internal Ethernet card.
I accessed the router through my browser, and clicked on "Attached Devices." Here is the list of Attached Devices:
# IP Address Device Name MAC Address
1 192.168.0.2 00:11:24:08:4c:88
2 192.168.0.3 00:14:51:6d:15:7c
3 192.168.0.5 00:0a:95:da:4f:24
The first two addresses are the two AE's, and the third is the internal Ethernet card of the PowerBook.
I ran "Netstat" under the Network Utility to get "Routing Table Information," and here is what I get (with some identifying info removed):
Internet:
Destination Gateway Flags Refs Use Netif Expire
default 192.168.0.1 UGSc 23 10 en0
127 localhost UCS 0 0 lo0
localhost localhost UH 11 2400 lo0
169.254 link#4 UCS 0 0 en0
192.168.0 link#4 UCS 3 0 en0
192.168.0.4 link#4 UHLW 1 1 en0
192.168.0.5 localhost UHS 0 14 lo0
Internet6:
Destination Gateway Flags Netif Expire
localhost link#1 UHL lo0
localhost Uc lo0
localhost link#1 UHL lo0
link#4 UC en0
justin-and-***** 0:a:95:da:4f:24 UHL lo0
**************bedro 0:14:51:6d:15:7c UHLW en0
link#5 UC en1
justin-and-***** 0:a:95:f3:3e:df UHL lo0
**************bedro 0:14:51:6d:15:7c UHLW en1
ff01:: localhost U lo0
ff02::%lo0 localhost UC lo0
ff02::%en0 link#4 UC en0
ff02::%en1 link#5 UC en1
Any ideas? Thanks again for your help.